If the options needed to change the SD card from "External" storage to "Internal" storage are not on your device, then that device either is using a version of Android that cannot make this change. Meaning prior to 6.0/"Marshmallow", or the device has been restricted by the manufacturer. If it is using an older version of Android you can see if it it is upgradable to the newer ones. However if it isn't, then there is nothing you can do without taking drastic steps (like rooting the device). Which is not something I can assist with. You would have to post any rooting questions in the "Rooting, Jailbreaking & Unlocking" forum. http/www.tomsguide.com/forum/forum-117.html
If the options needed to change the SD card from "External" storage to "Internal" storage are not on your device, then that device either is using a version of Android that cannot make this change. Meaning prior to 6.0/"Marshmallow", or the device has been restricted by the manufacturer. If it is using an older version of Android you can see if it it is upgradable to the newer ones. However if it isn't, then there is nothing you can do without taking drastic steps (like rooting the device). Which is not something I can assist with. You would have to post any rooting questions in the "Rooting, Jailbreaking & Unlocking" forum. http/www.tomsguide.com/forum/forum-117.html