If you signed up to be an Extension developer, but never completed on-boarding, and then progressed to the point where you should receive an invite to become an affiliate, the affiliate invite will not arrive.
For context, the partner, affiliate, and extension on-boarding leverage the same system. You can only have one type of on-boarding in progress at a time. The good thing, is that you are able to upgrade your on-boarding type (and do not need to re-take the tax interviews).
My suggestion is to complete the Extension developer on-boarding, which will then enable you to on-board as an affiliate.