HI I had exactly the same problem on a PC built for me by my local PC repairers. They suggested trying a scannow and then rebooting. I have just done this and lo and behold the deactivation watermark has gone. The system is still not activated though.
I have now found the activation key for the system and changed the key to that that had orignally been entered and I am now a happy bunny. It appears the 24H2 erased the original product key.