I have this exact same problem. I found a Reddit post recommending users post a support ticket, get immediately rejected (because it involves onboarding), and respond to the email they send you in order to get it sorted out.
I think this happened to me because a little while ago I went to look at making an extension and when I saw I had to fill out tax forms I didn’t do it at the time. After passing my Affiliate milestone it kicked me back to the Extension Developer Onboarding page, which I then completed successfully (all automated, no review needed).
jaykay907 appears to have gotten it sorted out because he got affiliate on September 14 going by a past Vod title (congrats dude). Will post back here what steps were taken to get it when I finally get it and time to get it if you have this issue.