@SquishyDowers This might be your saving grace!. As I had this problem carry over from FIFA19 (so I stopped playing for last 6 months)
Also had a problem using web app. Every time I swapped a player round in squad, I got a certain error msg...

Part 1:
Go to ‘My Games & Apps’ on ur Xbox,
Press the old ‘Select’ button on the FIFA20 game/box u can see,
Scroll down to ‘Saved Data’ & ‘Delete’ both Saved data,
(Don’t worry!, your Club, Level, etc, are saved on EA servers!)

Part 2:
Load up the companion Web App,
Go to your squad screen, (but don’t swap anything)
Create a new squad & leave it on that.
Now load up FIFA 20 & play!

I recommend starting @ Part 2 & try it without deleting the data 1st
If creating a new squad on the Web App does not solve your problem, start at Part 1 (hope this helps,)

Reply & let me (& others) know, if it solved your problem (good luck)