Hey!
It's Nathan here from the Trello Support Team!
It looks like Safari on your phone could be set to block Cookies which is likely what's causing this issue, can you have a go at following these steps to stop your phone blocking Cookies:
Once you've done this close the Trello app and reopen it and you should be able to sign in without any issues!
It looks like you're new here. Sign in or register to get started.