Assembly & not contained in equipment bill of material

What causes this issue?

You can only allocate an assembly in the header and operation of a
maintenance task list that is contained in the object BOM.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Select a suitable assembly using the structure explosion. Note that you
can only select assemblies from the structure explosion that have the
status "PM assembly" as BOM items.

Error message extract from SAP system. Copyright SAP SE.