I moved LR from desktop to laptop. My collections did not transfer. How can I get them back

Collections did not transfer — the usual cause is that you imported photos on the laptop. If so, that is the wrong thing to do, and the cause of your problem. Please follow the instructions exactly as given at the link given by ​@Rob_Cullen .

You posted in the Lightroom-Classic forum so I presume you are using Lightroom-Classic.

You copied the catalog (the .LRCAT file) to the laptop.

Did you also copy all the image files that are indexed in the catalog?

Are the image files on an external drive that you swap between computers?

For details about moving Lightroom-Classic to another computer see this link- and note the comprehensive eBook available in the linked article.-

Move Lightroom to a new computer? | The Lightroom Queen