Cannot create maintenance plan, I get the message "The task with the name '' and the creation name is not registered for use on this computer"
I have been having the same issue with SSMS 18 (preview 4) when trying to access any maintenance plan tasks. 

I opened this item on the feedback site to capture the bug. Feel free to upvote if this describes the issue you're experiencing.

For now I am just using 17.x versions of SSMS to do any maintenance plan related tasks, since you can have both installed side-by-side.  



The bug was acknowledged on 10/15 and it looks like MS is working on a fix, so hopefully the next release resolves the issue.