Why is the Instagram account I'm choosing to log into multiple accounts not working?
You can use one account's login to sign into up to 5 of your Instagram accounts at once. Any additional Instagram accounts you create will require separate login information or a single login for those accounts.
Keep in mind that if you'd like to choose an account that's already using another account's login, you'll have to remove it from single login first.
Note: Each of your accounts will also keep their individual passwords, which you can still use to log into each individual account.