Check the placement of the new account you added. Is there a Header before your account, not necessarily right before?

H - Header, then G- Group Accounts followed by T- Total Account. In the middle of the G accounts you could have one or more A - sub-group Accounts which must be followed by an S - Sub-group total account. After the S account you could have more G accounts or A accounts.

There is an hierarchical order for account types in the Chart of Accounts.  H-Header , A -Sub Account, S- Sub Account total, G-Group Account,  T - Total Account.  The error message will tell you where within the account number range the issue exists.   G accounts can stand by themselves where as an A account must be followed by a S account first before being followed by a G or Group Account