There might be multiple reasons for this. Please check the following:
Check that you are signed in to your iCloud account. See Apple’s support.
Check that you have Sync this iPad/iPhone enabled: Apple ID > iCloud > iCloud Drive > Sync This iPad/iPhone.
Check that the Concepts folder shows in the iCloud Drive section of the Files app.
If iCloud Drive still does not appear in your Concepts Gallery, restart your device. Once restarted, check the sidebar again.
If iCloud Drive still does not appear, please contact support via Ask Us Anything in the help menu or email support@concepts.app.