I bought a new MacBook and i tried to install my LightRoom 5 licence, but it does not work. I filled in the serial number etc. The technical support of Adobe advised me to go here to resolve the problem.
Well, if the operating system is OSX 10.13 or later, then Lightroom 5 may not run at all on that operating system.
Your choices would seem to be upgrade (pay money) to purchase Lightroom 6.14, or get the subscription (pay money) to Lightroom Classic CC. Please note that Lightroom 6.14 is not supported by Adobe, and will never be updated for any reason.