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That iPassword app has not been updated by the app developer for 14.5. That app currently appears to be abandoned, or maybe the password data was not migrated to a replacement app? What's happened with iPassword is unfortunately unclear.
Pending an update by the app developer, the only option here appears to be unloading all of your passwords, if you have a device running an iOS or iPadOS version prior to 14.5, and that's obviously and very clearly not a good one. An older iPhone or iPad device that's stuck at iOS 12 might be an option, if you can borrow one and re-set it to your Apple ID. Another bad option, yes.
There have been various previous reports of this:
app ipassword - Apple Community
iPassword App not working on iOS 14.5 - Apple Community
iPassword App not working on iOS 14.5 - Apple Community
ipassword and IOS 14.5 - Apple Community
iPassword - Apple Community
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