Please go to the below location and look for SLcahe & SLstore folder, trash both the folders.
Location: Library/Application Support/Adobe
Once done that, launch Lightroom it will ask for the activation details like serial #, please enter the same and activate it.
Close Lightroom, Re-open it and check if it prompts for same error.
Let us know if that helps.
I appreciate the swift response.
I'll try that and advise further.
Unfortunately by removing the folders I had to re install Lightroom 6. I am still receiving the same message. States. 'The Serial No.....is already in use by the number of maximum allowed computers. You need to deactivate another computer within 30 days to use this product'. I am only using this product on two computers as the data on my old I mac has now been completely erased.
I have rectified the Licensing issue.You should now be able to activate.
Please confirm once done.
Brilliant and many thanks. All works.