I have just purchased new Lightroom and according to account it supports Desktop installation. Need to migrate LR 5.7 to LrC. Old cat was saved in a different loc. LrC does nothing with Migrate from. Copying to default loc resulted in errors.

Sounds like you installed the wrong version of Lightroom. “Migrate” is a function of Lightroom, the version that stores images in the cloud. What you want is Lightroom Classic, that is the successor of your old Lightroom 5.7. After you have installed it, use ‘File - Open Catalog’, select your old catalog, let Lightroom Classic upgrade it and you are good to go.