I use my DJI pocket for many years now.
The device worked fine but whenever I connected to my phone the screen goes black but all functions working.
I used S10 plus, iphone 7, S21Ultra, Tablet. Mate 9

I use Samsung galaxy before and for almost 2 years had no issue with the device even the s10 is not supported for OP1 as the first excuse from support is the list of supported devices for OP1,  but even I use different phones, IOS and Android which are listed as the supported device still something happening.

Also, they remove the App from the Play store and many other places which u need to download it from their website. and most of the time getting errors that the app has Bug and won't run.

I check so many questions and post about this problem which they keep saying the same trash excuse about supported devices and ask people to make a video about it and leave it there.

I have DJI drone as well and when I face the same issues with the drone its was a same typical excuse and trash reason just to reject anything.

If I understood your message right, then the devices you have used have worked perfectly earlier on, am I right? If so, then did this problem occur after updating the Mimo app or your phone's/tablet's operating system?

I have unofficial threads that list compatible and non-compatible phone and tablet models, those threads also contain a troubleshooting section that could have tips on how to solve the problem. You can see the threads at https://forum.dji.com/forum.php?mod=viewthread&tid=235190 (for phones) and https://forum.dji.com/forum.php?mod=viewthread&tid=237518 (for tablets).

I had a similar problem with my father's tablet he borrowed to me - the controls worked, but the live feed only showed black screen or was frozen. It turned out to be a connection problem between the tablet and the Pocket 2 - after I put the tablet in a protective case, the connection became physically more stabilized and it started working as it should.

No matter what the cause, I hope you manage to solve the problem.

DJI just released a new version of the Fly App to make it compatible with Android 12.  If you have Android 12 on your Samsung S21 you can uninstall it and install the version directly from DJI's download website here.  Some users have commented that they had to turn off Pro Mode in mobile device camera settings if you have an Exynos processor.  DJI does have a tendency to update iOS versions first.  Because there are so many variations of the Android OS used by various OEMs it is harder for DJI to keep their Apps compatible with the Android versions.  To date no update has been issued for the Mimo App or the Go 4 App.  

For an answer on why DJI no longer updates the Fly App or Mimo App in the Play store see this thread for a response from a DJI R&D person.

Hopefully we will see an update to the Mimo and Go 4 Apps soon to make it compatible for more mobile devices using the Android OS.  

Could be Android 12 related. That was a big issue.
But you have it on the iPhone as well?

The Mimo app shows a black screen when connected to my Pixel 6 Pro running Android 12. Both direct connection by USB-C or connection via WiFi results in the black screen. I had the same issue with the Fly app until the recent update. Hopefully it's a similar issue with a similar fix for the Mimo app.