I was able to find my activation key and, as I suspected, it now says I have zero activations left.
My Resolve sometimes is asking me for the activation key for no reason - I type it and then it says "no activations left" (I have DR on two machines) confirming that it "found" my PC on the list. I call that a bug. I have the key on a text file, I copy and paste it. 20 seconds.
If you have ONE computer and DR is telling you there are zero activations left being used, I'd say someone else is using your activation key. The same computer should NOT count as another one. But I don't know how DR is serialising the machine. Have you changed some HW recently?
Sequoia: developers will work with the latest OS and software. They can't possibly support older versions unless there is a good reason for doing so - that is, some HW which only works with an older version.
That is why the first advice you're getting when asking IT for help is "update to the latest version".