Hi, @Richkm
Thank you for posting in Microsoft Q&A forum.
It seems it's an old issue that SCCM can not migrate the task sequence that contains steps from MDT.
We can use the export and import as a workaround:
- Export the TS from the source hierarchy using the "right-click/export" option. Just the TS itself, not the related packages
- Import the TS into the destination hierarchy
- For some reason all packages will be detected and re-attach except the "Setup Windows and ConfigMgr" action, so browse to that and update it
For your reference:
https://social.technet.microsoft.com/Forums/en-US/77a68a91-2c44-4313-abea-20ca7e0f2c6f/configmgr-task-sequence-migration-fails-due-to-mdt?forum=configmanagermigration
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