var options = {
                                fieldId: key
                            };
                            log.debug("options", options);
                            var fieldObj = objRecord.getField(options);

Options logs out

{"fieldId":"custbody_transaction_gateway"}

and

custbody_transaction_gateway

exists on the customer payment. but I get this error:

{"type":"unexpected error","code":"","details":"{\"type\":\"error.SuiteScriptError\",\"name\":\"SSS_MISSING_REQD_ARGUMENT\",\"message\":\"DeferredDynamicRecord.getField: Missing a required argument: fieldId\",\"stack\":[\"anonymous(N/serverRecordService)\",\"doPost(/SuiteScripts/CreateTransaction.js:186)\"],\"cause\":{\"type\":\"internal error\",\"code\":\"SSS_MISSING_REQD_ARGUMENT\",\"details\":\"DeferredDynamicRecord.getField: Missing a required argument: fieldId\",\"userEvent\":null,\"stackTrace\":[\"anonymous(N/serverRecordService)\",\"doPost(/SuiteScripts/CreateTransaction.js:186)\"],\"notifyOff\":false},\"id\":\"\",\"notifyOff\":false,\"userFacing\":false}"}	Remove

How could it be missing an argument? the options log.debu is showing the 1 and only argument needed for getField

sounds weird, try

objRecord.getField({ fieldId: "custbody_transaction_gateway" });

maybe try: var fieldObj = objRecord.getField({fieldId:options.fieldId});