You marked the account as a Control account in the account setup.

You typically use a control account for entries created in another application, such as Accounts Payable or Payroll. If you specify a General Ledger account as a control account, you can prevent users from creating entries to the account directly in General Ledger. This assures the control account remains in balance with the interfacing applications.

Change the control account setting to record entries to control accounts in General Ledger. To correct entries in a control account, you can record an entry in the correct interfacing application.