Finally managed it today !!!
Wiped the install from the MacBook Air and started afresh.
As previously mentioned I have signed out of Lightroom on my desktop MacPro which should free up a 'space' to allow installation on another computer.
Proceeded with the installation on the MacBook air (OS10.13.6).
This time, for reasons I can't explain, the dialogues were slightly different.
I was asked to put in the serial number of an earlier version of LR.
Fortunately I still had the details of LR4 which I duly entered and Adobe accepted that and then when requested I entered the LR6 serial number and hallelujah ! it worked a simple and trouble free install.
I am surprised though that I had never before been asked for an earlier serial number (ie the LR4 version).
Anyway it is now up and running and I am delighted to have my favourite quick and easy editing package back.
I'll still use Capture One on the MacPro for any professional work that requires intensive colour work etc. but going to LR is like a holiday afterwards.
I am surprised though that I had never before been asked for an earlier serial number (ie the LR4 version).
What happens when an older Lightroom version, such as LR4, is installed on a system for the first time, the registration number that you enter is saved to a sub-folder inside the LR installation libraries. When you subsequently install a later version which was purchased as an upgrade, the registration process needs the licence number of a "qualifying" product (such as the original LR4 number) as well of course as the registration number of the new product. However the registration process, after recognising that the new number is an upgrade licence, first checks your system drive to see if there's already a "qualifying" registration number in the designated sub-folder. If it finds one (which it should if it's never been erased), it doesn't need to ask you to enter the old number, so it just asks for the new number.
But if you install a later upgrade version on a system which didn't previously have a "qualifying" product installed (or was previously erased), in that circumstance you would be asked for both old and new numbers.