This error reports that you cannot complete the enclosure due to some problems to your geometry that does not permit to complete the boolean operation ( subtraction ) .
The Suface components cannot be used to create an enclosure, only solid parts are considered.
In your case if Enclosure fails it could be dependen on defects on you solid body. Verify that it is a solid without defects ( check with repair tools ). You can also try bolean operation manually, with Combine tool. You can apply it to smaller portion of your geometry in order to identify the corrupted parts and correct only it.
Luca
An error occurred while creating the enclosure (ansys.com)
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