Migration from Aperture to Lightroom problem!

"Attempting to migrate my Aperture file to Lightroom runs in to a problem, as the 'Import' button stays dimmed!"

There are several reports about this here but no solutions posted yet. In general, the Adobe-supplied Aperture/iPhoto Importer plugin is quite buggy and has caused lots of people lots of grief.

I recommend you give the Avalanche program a try -- a fair number of people have reported that it quickly and correctly imports Aperture libraries into LR:

https://cyme.io/avalanche-photo-conversion/

It costs €59, but how much do you value your time and your photos?

Have you referred to this how-to Adobe article https://helpx.adobe.com/lightroom-classic/help/migrate-photos-aperture.html

If not, you may find that you've missed a step. If you have followed this step by step you might need to migrate this manually by accessing the files within Aperture. You would do this by right-clicking on the Aperture library and then Show Package Contents. There you will find folders with the Master images and Edited images... you would need to copy the files to your drive and then import into your Lightroom Classic catalog. If you're unfamiliar with this process you may want to have someone help you. It not that it's hard to do - but it is not obvious either.