ERROR: This operation may take a while to complete

Many rules have been created but cannot create new rules. Is this a limitation to the free version? If so, I will consider paying. Otherwise, not interested.

How about a clue as to what makes you think you “cannot create new rules”?

OK, see that first word in the subject line? That subject line (following the colon) is the actual text presented by eM Client each time I attempt to create a new rule. It is NOT that it takes a long time to complete it simply fails even to start creating the rule.

To be clear, I am using the “Create rule from message” drop down menu choice.

Give some thought to uninstalling/re-installing eMC.
If you need some instructions, let me know, and I will post.