This has happened to me also. I made the booking on Ryanair's app using Google Pay and it got flagged as a OTA booking. I used their online form and had it unflagged two days later.

Ryanair is fighting a war against OTA bookings. They have a system which flags bookings as OTA bookings, and once your booking gets flagged you cannot do anything to that booking online. Even if you just want to check-in and take your flight without editing anything at all in the booking, you'll be asked to check-in at the airport.
From what I understand in the past some of those dodgy South-European OTAs would claim refunds on cancelled flights and never pass them on to the customers.

Just fill in the form, maybe attach Ryanair's confirmation email as OTA confirmation. Eventually you'll get your booking unflagged, after which you can apply for the refund. Initially they'll give you your refund only as a voucher, which you can use when logged into your account towards future Ryanair bookings. After that you need to fill in another form to get this credit converted into a actual refund to your original payment method.