Most of OEM's doesn't offer in gui option for adoptable storage. So you have to do it via ADB. Here is how
Here it is:
NOTE all data on your SD card will be lost.
Make sure USB debugging is enabled in developer options under settings and then connect your phone to PC via USB cable.
Open terminal or command line on PC ( make sure you have ADB installed if not then download minimal ADB and fastboot, you can find it on Google click ) type command "adb devices" to make sure phone is detected, then type "adb shell"
3.Type 'sm list-disks' to list the disks available, the result should be something like "disk:179:160"
- To make SD adoptable use command "sm partition disk:xxx:xxx private" where xxx:xxx is replaced with result from previous command, in some cases there is _ instead of : so watch out for that. If you want to use only half of SD as adoptable then use this command "sm partition disk:xxx:xxx mixed 50" and that's it.
If you want to move your data (pictures, videos etc) you can go to settings>storage>SD card>more and you will have there option "migrate data" hit it and let it go.