In order to disable the master key for an XRP Ledger account, you must configure another way to sign on your account. The process to do this is called, 'setting a regular key'. You must set a regular key before you are able to disable the master key on an account.
For example:
In this case, you can see that a separate account (ra433z72...) has been configured to sign for the main account. (rGVTc5FPz....)