Hi @heislervillefire,
Welcome to the Community. I'm here to lend a hand with setting up an income sub-account.
I appreciate you clarifying that the account types are the same since it's a requirement that the category type is the same. One option would be to enter the name of the parent account in the Enter parent account drop-down arrow and see if it appears.
Additionally, when a user encounters unexpected behavior in QuickBooks Online, it's often recommended to try general browser troubleshooting. Your browser will store cache and temporary internet files to speed up loading times, but too much can accumulate and affect processes. You can find additional information about clearing cache, as well as steps, at the following link: Delete Or Disable Cache And Temporary Internet Files In Your Web Browser.
I want to ensure this is taken care of for you, so please feel free to let me know by posting a comment below. Wishing you well.