Well it is going to depend on a few things. One, which version of Android you are running. If you are running anything less than "Marshmallow" then you won't be able to set the SD card as the default location for downloads and apps.
Now if you are running "Marshmallow", or you can upgrade to it, then the only other reason it would not work is that the manufacturer would have restricted that option on the device. Which does happen.
Check your OS version first. If it isn't "Marshmallow" see if you can upgrade to it. If you can, then you will need to start with a blank SD card. Why? Because changing the card to "Internal" storage will format and encode the card. Effectively wiping anything already on it, and making the cards data inaccessible in other devices.
If it lets you upgrade the OS, then try the following...
How to turn external SD card into internal storage on "Marshmallow".
1. Go to device “Settings”, then select “Storage”.
2. Select your "SD Card", then tap the “three-dot menu“ (top-right), now select “Settings” from in there.
3. Now select “Format as internal”, and then “Erase & Format”.
4. Your SD Card will now be formatted as internal storage.
5. Reboot your phone.
NOTE: If you don't reboot the phone, many things may not work correctly, so make sure you do.