If you receive a pop-up that you can't assign the job to a vehicle because the account has exceeded the Credit Limit, it is because the setting "Stop of Commited-> Credit Limit" is blocking job assignments for accounts over the credit limit.
You can disable that option by following these steps:
Hover over Administration.
Click Settings.
Go to Finance on the left side of the screen.
Go to Credit Control.
Disable the Stop of Commited-> Credit Limit option
Click Save on the top of the page.