Hi
https://www.youtube.com/watch?v=V9RPzOqJLFE
Here on this video this guys show how to fix this particular message. The video is related to Cisco webex but the procedure may apply to you case.
And pay attention to this information:
"
Firmware migration (Overview)
Multiplatform Phone (MPP) firmware enables the Cisco® IP Phone 6800, 7800, and 8800 Series to work with approved third-party call control systems as well as with Cisco Webex® Calling. The feature set provided by this firmware is not identical to that of the firmware designed and built for use with Cisco Unified Communications Manager, but there are many similarities. For the remainder of this document, we will refer to firmware designed and built for use with Cisco Unified Communications Manager, as “Enterprise firmware.” MPP phone firmware does not work and is not supported on Cisco Unified Communications Manager.
MPP firmware can run on certain models of the 6800, 7800, and 8800 series. However, only the 7800 and 8800 series have the capability to run either MPP firmware or Enterprise firmware. If you have the appropriate license, you can migrate between the MPP or Enterprise firmware on the 7800 and 8800. Note: 7800 firmware migration requires that the 7800 model is the correct hardware version."
Make sure you are running the MPP firmware and appropriate license.
https://www.cisco.com/c/en/us/products/collateral/collaboration-endpoints/unified-ip-phone-7800-series/guide-c07-742786.html
