It has nothing to do with file manager or any other app. The reason you can't is because you are running an older version of Android. At present only "Marshmallow" will all for turning the SD card "Internal" rather than "External". You are running KitKat. You can see if your tablet will allow you to update to "Marshmallow", but there are no guarantees.
The only other option you could try is to root the device. That I can't advise you on. You can, however, visit the "Rooting, Jailbreaking & Unlocking" Forum.
http
/www.tomsguide.com/forum/forum-117.html And ask questions there. Although that forum is for phones, the principle is the same as they are all running versions of Android.
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