When sending a transaction to MYOB the below error appears.

This may be because someone has opened myob using a wrong shortcut (myobp.exe)

All MYOB users MUST use the shortcut which points to the application "wms-myob.exe" to prevent this error message.

Otherwise, the person who is using myobp.exe must logout from MYOB each time in order to WMS to communicate with MYOB. 

Detail Steps

How to find myobp.exe user

You can find out who is using the myobp.exe wrong shortcut in Task Manager 

Write click on the Taskbar and select Task Manager ( Ctrl+Alt+Delete ) 

Select Processes Tab and  find out who is using Myobp.exe.

How to change the shortcut. 

Ask this user to follow these steps to create a correct wms-myob.exe shortcut on his desktop.

Delete the incorrect Icon from the desktop or Start Menu.

Go to the Myob software installation folder. This is usually in C:\Premier19 or C:\Enterprise

If you can't find the saved location please contact your IT administrator.

Inside this myob folder usually there are two *.exe file

Right click on the icon called wms-myob.exe > Send To > Desktop (create shortcut) 

If you are unable to find wms-myob.exe please copy myobp.exe and rename as wms-myob.exe and send to public desktop.

Everytime you MUST use this wms-myob shortcut to open Myob.

Related Information

Another reason for this to happen is MYOB crashed you need to release locked records in WMS. File > Active Users > Remove Locks/Remove All.