I'm not sure, but it could be because the "more than 5% ownership" rule may be the reason. The best I can find on that is at https://www.irafinancialgroup.com/wp/what-you-need-to-know-about-making-contributions-to-a-solo-401k...
So if you own more than 5% of the company and did not take an RMD, that "may" be why you can't contribute.
