It seems Cash App officially does not support Graphene OS.
I called Cash App support for help with the error "We couldn't verify that this account belongs to you." on Graphene OS. As a troubleshooting step they wanted me to create a new Cash App account, and that is when the app directly told me "This device or software isn't supported \n Cash app doesn't support the use of emulators or other software programs". And after consulting with their specialist, my support person said that I will have to use another device.