Android 7.1.1 showing insuficient space even with plenty of space in SD card formatted as internal
You have to keep your internal storage empty because all the apps which you download from play store gets into your internal storage not in your external storage, even if you buy a sd card of 128GB or 256 GB you will not be able to install apps if you have full internal storage

Suggestion: Move all your photos ,videos, downloads to your SD Card
manually and  especially delete that .Thumbnails folder(Hidden Folder)
in DCIM in internal storage.