You may not need to add a header, you may just need to renumber the header below the account you added so it's number is above the number of the account you added.

The account types on the Chart of Accounts, General Ledger show the order of the types of accounts.  You must have a Group Heading at the top of each group.  If you have a Subgroup Account or Group Account or Subgroup Total above a Group Heading, you have done it wrong.  You must have nothing above the Group Heading or a Group Total.

The only way we can give you more specific help is for you to tell us what accounts you are working with.  You can also open the Chart of Accounts and hit the Red Checkmark on the toolbar to narrow down the issue and give you a message.  Those messages are helpful to us but it won't give us all the information as we do not know what your numbering of accounts is above and below the number.

If you have not posted to the account, you can delete it again to start over, or renumber it so it is below the Group Header or if there is a Group Total above where you would like to have the account grouped, you can change the number to one above the Group Total.

I assume you have something like the below image (ignore the fact that my accounts have 5 digits and your's might only have 4).  The blue highlighted account is the problem as indicated in the message.  Note the H (Header) account is below and the T or Total account is above.  That's wrong.  So renumber the new account (if you have not posted to it) so it is in the above or below group or renumber the Header or Total to move it to include the new account.  Because my offending account is called Other Services, I could renumber it to 40900 to group it in the above group above Total Revenue: Services.  This would fix the problem.