First, ensure your username and password are 100% correct & you do NOT have two-factor authentication turned on your IG account.  

If you received a “Checkpoint Error” login to your Instagram account from your phone or computer to verify “It was me” in the security verification prompt.

From your Instagram account on either mobile or computer select “It was me” in the prompted security verification on your screen.

After verifying “It was me” from your Instagram account go back to your dashboard and re-add the account by selecting “ADD ACCOUNT”