Bevelling not working on Back Slots

Its probably because of the face angle. You may need to make the slots a little deeper or change the angle on the modifier.
Can you share a screen shot of the back slots area?

And the remember the usual suspects. Be sure to have your scale applied. Be sure to remove double vertices. Be sure you have consistent normals. Check for overlapping faces and such.

In edit mode, press A to select all verts, press M and select by distance.

I can't see the link anywhere. Did you post it here or somewhere else?

One of the things that could be happening is your object scale. Be sure you applied the scale of your object. After that, try adjusting your bevel amount. In the video you have it set to 30.1 cm, so that may be too small or perhaps too big, depending on the scale of your object. That's why it's important to model to scale or not have the scale values out of whack. If you're still having trouble, you can share a link with your file so we can help do some troubleshooting. 

You may have double verts. On the Bevel Modifier, Clamp Overlap is enabled. This stops the bevel from overlapping the next verts along the edges. Uncheck this and try again. If it now bevels but gives a horrible looking mesh overlap, undo the bevel (CTRL+Z), Press A to select all, Press M and select ByDistance, This will remove any double verts. 
Re-enable Clamp Overlap and try the Bevel again.