To fix this issue do the following:

Settings  Installed Apps
To top right corner tap on this window click the SETTINGS icon (3 vertical dots)
Click Show All Apps

Now Scroll Down and Search for "File Manager"
It should be ENABLED (if not then tap on Disabled and Toggle to ENABLE it)
Now tap on Clear Data and Clear All Data

REBOOT the phone (This step is necessary to solve this issue)

After Reboot again go back into Installed Apps and search for
"com.android.fileexplorer.FileExplorer..." Tap on it

AUTOSTART must be Enabled
Now Tap Permissions -- Enable all Permissions
Reboot Phone

After Reboot try moving some files from Internal Storage to SD Card it should be solved.
If not then try both the above steps once again.