1. Disconnect Internet connection and force quit all instances of Parallels 10.
2. Re-run Parallels 10
3. On the Trial pop-up, click enter activation key.
4. Type any combo of letters and numbers, once you see a green check, press activate or continue.
5. It will load and if it asks you to continue as trial or activate again, DO STEP 4 AGAIN!!
6. Once all activated and loaded, FORCE QUIT ALL INSTANCES OF PARALLELS 10.
7. Go>Go To Folder ( Or, SHIFT+COMMAND+G) /Library/Preferences/Parallels/licenses.xml AND MOVE TO TRASH ( It should already be highlighted or selected ) ( Permission must be given, enter admin password. )