Either you have more tools than you do racks, or the tools don't fit in the rack, you'll need to revert to Manual Tool Change mode to utilize those tools not allocated to a slot. 

If you've just tried to swap out the tools you likely encountered this error. Tool to unload is not set to any slot

The only way you'll be able to utilize the tools that are not allocated to a slot is:

Change to Manual Tool Change under tool changer

• You'll still want to have Manual Tool Change location setup for instances like this
• Your previous settings and offsets on the tool setter will remain in effect.

Initiate a tool change utilizing MDI and T#M6 where # is the tool number you want

Zero Z according to the toolpath programming

Run your tool path

Initiate a tool change to a tool dedicated to a slot

Zero Z (If you skip this, any subsequent passes may be high or low)

Change back to Linear Tool Change 2

The good thing is that Masso is smart enough to search your entire g-code when starting a program for any tool change errors and will trigger a warning before beginning, so this isn't going to "just happen"

What is not mentioned in the video is the need to probe z after changing. The video was originated from Tool to unload is not set to any slot.