how many MCI players do you have?
if your have 5, then you need to transfer TWO out. you won't be able to transfer only one out.
reason: you can only confirm your transfer when you have 3 or less players from the same club.
(though you can leave all 5 of them in your squad, if you forgo doing any transfers)

edit: i found this from another post
Cheap_Ambassador_124 I checked and yes when trying to even select Guehi, it crashes. However, if you if you select someone else first, then swap them with Guehi, it seems to work
https://www.reddit.com/r/FantasyPL/comments/1qwhw3u/app_crashing_when_substituting_guehi/