You need to make sure that the folder which cotains your catalog file has WRITE permission. Go into your operating system and change the permission on this folder to WRITE.
Thank you for your help. I now get this message. "Lightroom cannot launch with this catalog. It is either on a network volume or on a volume on which Lightroom cannot save changes." - my external hard drive does not show an option for 'write permissions'.. have you got another idea? Frustrating
You are not supposed to change the permissions on the drive, you are supposed to change the permissions on the folder that contains the catalog file.
Is the catalog file on your external drive, or on the internal drive?