After saving some parts from the assembly scene with an external link, open the externally linked parts separately.
Within a single external link part, you cannot create an assembly.
Causes
Example: Open externally linked parts separately from the assembly diagram on the left. (Right figure)
You cannot create an assembly within a single part of the right-figure.
The following error message is displayed:
Parts saved by external links are linked to assembly diagrams.
When you assemble on the external link side, you will see a message that cannot be assembled because it is used in the assembly scene.
How to deal
[1] Unlink the external linked part within the assembly scene.
[2] After unlinking, create assemble the part file that individually had been opened.

[3] After deleting the original data that was externally linked to the assembly scene, insert it again.
When inserting, be sure to enable [Insert as Link] in Insert Settings.
If you want to create a separate part and replace the original external link without cutting, see here.
※Ironcad 2023 and later