Hi @Mchri,
Thanks for posting.
By deault after you click the "Go work", the Flow Mobile App would navigate to a sign-in page.
Besides, do you have any other Microsoft Mobile App used, like OutLook Mobile, OneNote Mobile? If yes, are you able to sign-in with them?
Two things you may take a try:
1. Verify your Device network (Proxy or firewall settings), make sure there is no policies configured to block the Sign-in page,
2. Uninstall it and re-install the Mobile App to check,
Regards,
Michael