Hello Jigglypuff_Of_Doom,
Thank you for using Apple Support Communities. Based on your post, you can't set up Express Transit with Apple Pay.
If you can't select the Express Transit option and it defaults back to none, try to remove the card and add it back again:
Use Express Transit with Apple Pay Under section, 'Remove your Express Transit card' and then add it back to test it out.
Use section, 'Pay for your ride with Express Transit' for the transition itself.
Should it persist, do you have any other issues using Apple Pay?
Have you considered updating to iOS 15, as suggested in the resource above to see if that helps out? (Don't forget to backup first!)
Hoping to hear back, cheers!