"Tool to unload is not set to any slot" 

This happens because you have a tool loaded that is not dedicated to a slot. 

There are several courses of action to rectify the issue. 

1: Change to manual tool change, and command a tool change using the tXm6 command where X is the tool number. 

2: Go to the F4 screen and add a slot to the tool (must be an empty slot), then use the previous tXm6 command. 

• You could remove the tool from the spindle using MTC Button, and set the current tool loaded according to Masso
  to a non-existent but defined slot location and use the tool change command. 

The best way to alleviate this problem is by not making changes to your F4 screen as it relates to tool #'s and tool slots.

If you find yourself having to constantly reassign slots, it might be time to get more tool holders and tool racks.