Your pass is valid on that train. Just ask a reservation for Interrail.

Deutsche Bahn should also be able to book this train, but only at the ticket office or via phone.

Ive never been interrailing before so how does it work? Do I need to show them proof I have an interrail pass?

In the train certainly. At the ticket office, sometimes (depends on the railway company). When ordering via phone: hardly ever.

Really appreciate the help. I have my pass on the phone app so will I need to add this journey to the phone app or will the ticket from Sofia and showing them the pass on the app be sufficient?

Normally showing the pass should be enough.

Until now, I've only heard from RENFE ticket offices that they want to see a pass with the journey already added. Which makes me wonder if they also ask that for a paper pass, which is a definite no-no for travel on a future date. On a mobile pass, you can always cancel future travel days.

About to use again in ESpana-next week-but last times I was there and on a pass (pre-covid) they always, always wanted to see the pass and validty-but I think I always had continuous passes then, so do not know about passdays. They must have very strict instructions that noone could use pass without using passdays for trips.

It might be useful to take a look at the video on this page as it explains pass activation and adding journeys. 

Not only RENFE does like that. Also Trenitalia wants to see that you added the trip to your pass when you buy a reservation there with mobile pass...

Also in countries like the UK that can’t read the QR code the text needs to show the train details as well as the travel day validity information. 

It is my understanding that without moving the train to the pass it is not a valid ticket to travel, even if you have a relevant QR code for the day..

I also believe that the actual train details are not shown in the QR code and inspectors read the text to validate the train has been loaded.

I know from personal experience that the French gangs of revenue officers that seemed to board every regional train in the Cote d’azur were seriously diligent that all info was correct, especially that the specific train was shown, not just that the pass travel day was activated. Fortunately mine were present, but I felt sure they would have loaded me with a 50 euro fine if there had been any deviation from their checklist, as many others found out - no excuses were accepted and these were gangs of 6 to 10 inspectors.

On one train from Cannes to Monte Carlo the train was visited twice by different gangs and both insisted on checking the pass thoroughly.

On the few occasions that I've bought reservations at a Trenitalia ticket office, they didn't ask to see a valid travel day, they just wanted to see the pass data (name, pass number etc.).