: Sometimes the profile for AltStore itself is the one that is invalid. Connect your device to your computer and use AltServer to Install AltStore again. This will overwrite the old profile without deleting your apps.
: In the AltStore app, go to My Apps and tap the App IDs button in the top left. If you have multiple IDs for the same app, you may need to wait for them to expire (up to 7 days) before a new valid profile can be created. Advanced Fixes
If you encounter this error on your computer or iPhone, try these steps in order: