When you are moving leads between lists, the lists must have identical fields (the order of fields doesn't matter). If the fields are not identical, the migration will fail.
The fix: make the fields match
Field definitions can be edited at any time, so the quickest fix is to align the two lists:
Open both lists' configuration pages (Manage -> Lists -> Settings) in two browser tabs.
Compare the field names and types side by side.
Edit whichever list is missing a field — or has a differently named or typed one — until both lists match exactly (names and types).
Run the migration again — it will now succeed.
This works even when the lists belong to different clients, and you can restore any field differences after the migration if you need them.
To get a more detailed error message, containing information on exactly what fields you need to fix, create a pipeline automation and run it in test mode.