Which Date? Maybe you search to far in advance. My experience regarding trains in the UK is you can reserve them just 45 or 60days in advance :/ 

Which Route? Maybe you try to reserve a non reservable route.

I searched London Kings Cross to Edinburgh route on July 1, 2023, I reckon it was the reason that I tried to reserve too far in advance. I just modified the departure date to May 20, and then I could see the ticket options. I am a bit confused about the options though, what's the difference among all the options? Isn't “1st Class Advance single” and “Advance Single (1st Class)” the same thing? What about off-peak and anytime? If I pick 11am, would that be considered as off-peak? Thank you :)

Which Date? Maybe you search to far in advance. My experience regarding trains in the UK is you can reserve them just 45 or 60days in advance :/ 

Which Route? Maybe you try to reserve a non reservable route.

I searched London Kings Cross to Edinburgh route on July 1, 2023, I reckon it was the reason that I tried to reserve too far in advance. I just modified the departure date to May 20, and then I could see the ticket options. I am a bit confused about the options though, what's the difference among all the options? Isn't “1st Class Advance single” and “Advance Single (1st Class)” the same thing? What about off-peak and anytime? If I pick 11am, would that be considered as off-peak? Thank you :)

 

The options you see doesnt matter for your Railpass. This Reservation option comes from the difficult british ticketing system with Advance fares (cheap advance tickets), off-peek fares (can travel only out of rush hour) and Anytime fares (you can travel on any train you want) 

For your reservation you just click on one of it for the class you wanna travel Just note even as you can reserve 1st class you still need than a 1st class railpass :) 

For Standard class you can use your 1st class pass or a 2nd class pass 

From time to time travelers think the reservation is their Upgrade to a higher travelclass :/ 

Which Date? Maybe you search to far in advance. My experience regarding trains in the UK is you can reserve them just 45 or 60days in advance :/ 

Which Route? Maybe you try to reserve a non reservable route.

I searched London Kings Cross to Edinburgh route on July 1, 2023, I reckon it was the reason that I tried to reserve too far in advance. I just modified the departure date to May 20, and then I could see the ticket options. I am a bit confused about the options though, what's the difference among all the options? Isn't “1st Class Advance single” and “Advance Single (1st Class)” the same thing? What about off-peak and anytime? If I pick 11am, would that be considered as off-peak? Thank you :)

 

The options you see doesnt matter for your Railpass. This Reservation option comes from the difficult british ticketing system with Advance fares (cheap advance tickets), off-peek fares (can travel only out of rush hour) and Anytime fares (you can travel on any train you want) 

For your reservation you just click on one of it for the class you wanna travel Just note even as you can reserve 1st class you still need than a 1st class railpass :) 

For Standard class you can use your 1st class pass or a 2nd class pass 

From time to time travelers think the reservation is their Upgrade to a higher travelclass :/ 

Thanks so much for your answer @seewulf :) We do have 1st Class Eurail passes I will book the reservations maybe in a month time or something then :) 

12 weeks ahead is the standard booking window in UK although there can be variations. Weekends are when most engineering work takes place and timetables can be released much later due to this.

For reservations on trains run by LNER only, Direct trains from London to Leeds, York, Newcastle, Edinburgh, Aberdeen and Inverness: You can choose specific seats by using their own website/app.

https://www.lner.co.uk/travel-information/make-a-reservation/

GWR is the only online reservation source for all other train companies.

Thanks so much @Al_G :) If I book reservations for the West Highland Line from Glasgow to Mallaig return, can I use LNER’s official website or should I book on GWR? 

I am not sure that those (day)trains are reservable, and I also don’t think it has 1st class. It’s usually done by very tiny regional train sets.

I tried to check via GWR, but it doesn’t give any reservation option for any of their routes (even their own). Maybe a technical issue?

Anyway, did Scotland last year, and I only reserved the long stretches (London-Scotland and back). In Scotland itself, it really isn’t necessary. Plenty of space. If trains looks crowded in end stations like Glasgow, just walk up to the front of the train, will be much calmer there. People are lazy, and usually step on the trains via the first doors they see.

Seek out the outer ends if you want a quiet ride, a tip that’s always applicable for whichever (non-reservable) train. 

Thanks so much for the tips @BrendanDB :)

I just tried to book the seat reservations again on GWR, it was very strange… I followed the steps of this post. 

Then when I clicked the “Add reservation” button of step 7, the page doesn't have a response. it looks like this 

 So I ticked that box of “I would like to reserve a bike space” then it worked. I am still confused if my seat reservation was successful or was it just for the bike reservation (I don't have a bike) 

I also changed to earlier dates to test, it had the same response…

Seat reservations do not exist for that route and many others.

Edinburgh - Glasgow has frequent trains across multiple routes, reservations are not needed.

oh that makes sense now. Thank you so much @Al_G