The Rank field is a Dropdown that shows the Ranks that you have created in the Ranks Settings (Affiliates > Settings > Ranks). This will only appear once you have created some Ranks. Then, you can choose a Rank from that list and save to apply that Rank to the affiliate.
What version of AffiliateWP and AffiliateWP Ranks are you using? Also, do you have any other AffiliateWP add-ons active, and if so which ones?
Also, have you tried deactivating and reactivating the Ranks plugin?
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