Hi S-L_H,
Welcome to the Apple Community.
Sorry I'm no expert on the use of migration expert.
Apple support have articles online, that may help:
Transfer your information to Mac from another computer or device – Apple Support
Eg: says:
"If you didn’t transfer your information to your Mac when you first set it up,
use Migration Assistant anytime to transfer your documents, apps, user accounts and settings.
You can transfer your information from a Mac or PC, or from a disk or Time Machine backup".
Move your content to a new Mac - Apple Support
Eg: says that your Mac has to have all recent updates installed,
in order to be ready to use migration assistant.
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More info: if needed:
Where on my Mac are the files I transferred from my PC? – Apple Support
macOS User Guide – Apple Support
(To view various topics leading to detailed articles:
Click on "Table of Contents" at the top of the Welcome page that opens from the link.
Hope this helps :-)