back camera are not working

There's not much detail in the question, so here's how I'd work through a dead rear camera on the Note 8, easiest first.

Start with software. Go to Settings > Apps > Manage apps > Camera, then Force stop and Clear data (this wipes the camera app's cache and settings, not your photos). A corrupt cache is one of the most common causes of a camera that suddenly stops working.

Next, boot into Safe Mode: hold the power button, then long-press "Power off" until you're offered Safe mode. If the rear camera works there, a third-party app is interfering -- uninstall any camera apps you added recently.

Also check for a system update. Xiaomi has fixed camera bugs in MIUI updates before, so make sure the phone is on the latest build available for it.

If every rear lens is dead: the Note 8 has four rear cameras, and they all connect through the mainboard. When all of them fail but the front camera still works, the usual hardware suspects are a loose camera flex connector or a failed main (48 MP) camera module -- often after a drop.

For the hardware check, the back cover is glued on, so warm the edges gently (a hair dryer on low works), then work around the seam with a thin plastic pry tool -- avoid metal tools anywhere near the battery. Once inside, disconnect the battery first, then pop the camera connectors off and reseat them firmly. Reseating alone fixes a surprising number of these.

If reseating changes nothing, the main camera module itself has likely failed. Replacement modules for this model are inexpensive, and the swap is straightforward once the phone is open -- it just plugs in.

If you can add a little more detail -- does the app crash, show black, or give an error, and did this start after a drop or an update -- I can help narrow it down further.