This notification is displayed when your MT5-based trading account balance is less than 1 USD (such are the conditions specifically for MT5-based accounts).
Trading will be enabled again after depositing into your trading account: click to Deposit Funds.
It may take some time to update the information on the server after a successful deposit. If you still see the notification in the terminal, you can restart the trading terminal or re-authorise your trading account to force the update.