There is a similar post that has been marked as answered. However the "answer" was rebooting the computer.
Reposting as unanswered till I see an SPR number or a real resolution as to why this is happening and how to fix it.
Basically for a while I have been running into parts that are not "selectable".
Frequently this occurs after:
recently inserting a part into an assembly
undoing a delete of a component in an assembly
dissolving a subassembly
We are using SW 2014 in production, and my "tricks" have been to supress/unsupress the problem part, or set lightweight/set resolved. However, in testing 2018 in advance of our production upgrade, not only does it seem that my "tricks" no longer work (short of closing and reopening the document), it seems that the problem is more frequent as well.