There are no direct trains on this connection. You need to change trains in London. 

Manchester - London has only optional reservations, so you don't necessarily have to book a seat. These are free btw, on domestic British trains. GWR can book seat reservations on most day trains in the UK. For free. 

Eurostar (London - Paris) should be booked asap. It has on'y limited seats for pass holders. When  are you travelling exactly ? 

Seat reservations for Eurostar can be found here:

https://www.b-europe.com/EN/Kopen/Pass#TravelWish

For which you need a pass cover number, generated here:

Press "later" or "earlier", if you don't find trains for your specific travel time and date. 

I know direct is not an option, though I am trying to book with my Eurail pass. Despite there being routes, I am unable to reserve seats which is the issue I am trying to resolve. Might you have any suggestions? I’ll be traveling toward the end of April.

And thank you for your response and help!

@BrendanDB has give you information where to make the reservations. Have you tried to make your reservation according to the information given by him?

@BrendanDB @AnnaB I’m looking at Manchester to London now (rather than London to Manchester) and GWR is showing each ticket as £64. When I look through Eurail, it says that a seat reservation is required for the route.

If you have a valid travel day for that day, you can get a free reservation either at the train station or online indeed through GWR (but not through their ticket search engine). Be sure to follow the steps listed here: 

@Schelte That worked! Thank you so so much!

The Eurail timetable is not reliable, also for this kind of information, and should not be relied on.

All day trains in GB have either optional reservations or don't have reservations at all.