Accounts are required to meet the Initial Margin per contract to carry a newly opened position (as of that days trading session) through the close of trading. You must have the full Initial Margin per contract in your account.
Example: You have a Net Liquidating Value of $20,000, and currently are long 2 Emini S&P contracts. The Emini S&P has an Initial Margin requirement of $12,320 (as of 9/01/2019). 2 ES contracts have an Initial Margin requirement of $24,640 (2 x $12,320). You must reduce your position by 1 contracts in order to carry through the close ($12,320).