Rear wheel squeek Motorcycle(video link)

Could be a few things... It's not unusual to get a slight rubbing noise from new brakes, but this sounds like something that warrants at least further investigation, as it's a bit more than just a new pad to disk contact noise.

Firstly though.. just check that the chain is not far too tight. This can cause similar noises sometimes especially if the chain has tight spots. The load from the chain can really pull on the rear sprocket if tight & load up the rubber sprocket/hub cushion which can bind. If the chain is too tight, adjust it to the correct specs.

The new pads could also be binding causing this noise, especially if the caliper/pads are not floating/sliding freely, or the pistons are sticking. Although this should have been checked and lubricated properly when the brakes were done.

Try 'very carefully' to push the pads/pistons back into the brake caliper a little bit with a small prybar or screwdriver, just to create a small gap between pads and disk.. Is the noise still there? If the noise has gone then the brakes need looking at again as they are binding which is giving you this noise. They may not be operating correctly, or as smoothly as they should be.

If the noise is still there after you've checked the chain and made sure that the pads aren't binding on the disk, then it could be a bearing that's causing the noise. Brand new bearings don't normally give issues, but they can be installed incorrectly, sometimes ending up misaligned or overcompressed etc which means that they don't rotate smoothly.

Castle nut is too tight. Loosen it up and see if the squeal goes away.

Probably this occurs due to "dried out" bearings and not due the disk brake OR the chain. A good way to confirm this is to raise the rear wheel, mark a point around its circumference and slowly rotate it. If the issue is the disc brake the noise will be heard at specific position of the wheel. You can do the same with the chain, just clean a link and rotate the wheel checking if the noise occurs at specific chain position. If none of these stand, slowly rotate the wheel and expect for a small "step" (or more) in it's movement. Finally remove the wheel axle, lube it carefully with some grease, check the bearings by rotating them with your finger (and confirm no "steps" on their rotation) apply grease as well on them and reassemble. Otherwise change bearings and along with them their seals.