This started happening to me because of iMessage but it was happening to my buddy because of reprovision. I was on 12.4 and he’s on 13.something, both using unc0ver+reprovision.

For my phone I just had to sign out of iMessage and FaceTime on every device that was using my Apple ID, including my MacBook and old iPads and iPhones I never use, reboot them all, then sign back into them one at a time. Haven’t had the message pop up since.

For his phone we had to do all of the following before the message stopped appearing:

• manually sign unc0ver with reprovision to start your 7 days over
• delete all the certificates from reprovision (I’m not near my iPhone with reprovision but I believe it’s Troubleshooting->Manage Certicates)
• sign out of your Apple ID in reprovision
• delete reprovision
• in the phone’s Settings app go into iMessage and click the toggle switch to turn it off, do the same for FaceTime, then sign out of iCloud by clicking your name at the top of Settings and scroll to the bottom to click “Sign Out”. I think it asks you to disable Find My as well.
• run a clean with iCleaner Pro
• hard reboot the phone (hold Home+Power buttons until you see the the  logo)
• enter your pin, launch Settings, sign back into iCloud and then enable iMessage, FaceTime, and Find My
• don’t leave Settings and don’t launch any other apps, wait 5 minutes and then turn on airplane mode, reboot the phone normally
• enter your pin, launch unc0ver, let the phone sit for at least 3 minutes
• re-jailbreak, turn off Airplane mode after the respring
• reinstall reprovision and sign back in with your Apple ID
• re-sign unc0ver with reprovision