cannot access phone directories using adb shell after upgrading to android 11
On Android 11 sdcard path seem to have changed. The path /mnt/sdcard still exists but via adb you don't the permissions to access this path.
But there are alternatives. On the an emulator running Android 11 I can see for example path /sdcard. This path however is a soft link to /storage/self/primary which again is a soft-link to /storage/emulated/0.
Hence you have the following alternatives:

/sdcard
/storage/self/primary
/storage/emulated/0