Error closing order 138 in MQL4 can refer to an issue encountered with the Metatrader 4 (MT4) platform. This error occurs when a trader is attempting to close an order and the system fails to do so. This issue can be caused by several factors such as network connection problems, broker server issues, or an inability to process the transaction for some other reason. There are several solutions that traders can attempt to resolve the issue, such as restarting the MT4 platform, checking the broker connection, or logging out and back into the server. If the issue persists, it is recommended to contact the broker for further assistance.