I am getting the same message. I'm OK at the mo, because I still have plenty of space to add new items. But I am thinking ahead and I will hit the maximum allowed, because of this error, at some point.
I'm nearly sure it is counting stock that I have now deleted, so I can't untick them to remove them from the promotion.
I have to phone eBay sometime about something else, so I will mention this. If they say anything useful, I will try to remember to post it here. But in case I forget, it might be worth contacting eBay about this. At the risk of being patronising, my method of getting through to speak to someone is to click on their "Help & Contact" at the top of every page. Then I scroll to the bottom of the page and click "Contact us". That will put you in touch with their chat bot, which is as much use as a chocolate teapot. You have to click on options in their system about what the issue is, none of which will apply. But just type something like "I need to talk to an advisor". Sometimes it takes a few goes, but that will lead them to ask if you want an advisor to call you back, or you can do it via "chat" if you prefer.
HTH
