There are two possible reasons for this to happen: one is when you unknowingly get logged out of Shopify, and the other is when you choose to upgrade the app (or start the free trial) and Shopify is still waiting for your approval to the subscription charge that will be incurred at the end of the trial unless you uninstall the app before then.
To troubleshoot, you can try the following steps:
Log out of your Shopify admin, then log back into Shopify and access the app through the Apps section.
Sign in to your Shopify admin using a different browser and access the app via the Apps section.
Log out of your Shopify admin, clear your browser's cache, sign back in to Shopify, and then access the app via the Apps section.
If you recently opted to upgrade the app, click the "Approve" button to proceed.
