Happy to help troubleshoot this! I have a few questions that'll help diagnose the issue:
Would you mind clarifying what you mean by "app?" - Is this the Shopify mobile app or your mobile browser?
When you say you couldn't log in, what do you mean? Are you seeing an error, or something else?
Can you provide a screenshot of what you see?
Are you able to replicate this on a desktop device?
Trevor | Community Moderator @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog