Strange. We got one other report like that. My best guess is that there are certain Cash App accounts which are restricted from sending ANY bitcoin at all... for example, I have access to a Cash App account, and was just able to send to a Rizful lightning address without any problems. The other user who reported a problem, it turned out (we asked him to do some tests) -- he wasn't able to send Bitcoin at all -- not onchain, not lightning, nothing!!