I'm not sure what you mean by "Copied over my catalogue"
In LR CC go to File -> Migrate Lightroom Catalogue. I would then browse to your original catalogue.
Here is a page on how Adobe recommends preparing for and migrating from Classic to CC
Migrate photos and videos from Lightroom Classic CC to Lightroom CC
Strange. After you copied the catalog to your new machine have you opened it in Classic? As a first step, I'd try that. Beyond that, I don't know...
You‘d need to upgrade from Lightroom 6 to Lightroom Classic CC (the traditional desktop folder based Lightroom) before migrating a catalog to (the new cloud based) Lightroom CC.
Got around to upgrading to Lightroom 6 and still no luck. I'm still getting the error message "the lightroom classic catalog appears to be an exported catalog or has been copied"
My current catalog has missing files. Is that what is causing the problem? or can I migrate previews and then re-link the originals?