I have a textbook android tablet and a SD card but when I try to download apps like viber and other apps they say I have insufficient space and my tablet is saying storage space is running out some systems may not work
The storage used to store apps is internal storage ,android doesn't allow apps to be installed on the SDcard so no matter how big is your SDcard your tablette will install apps in the tablet's internal storage. You can move apps to SD card using app2SD or in the phone settings but only apps that allows move to SD will be movable if the app doesn't allow app2SD you can't move it.
There is some advanced work around to use an ext4 partition on your SD card as a /data parttion but it needs to be anabled at the kernel level.
Android versions less than 6.0 don't natively support using an external sdcard to 'extend' the internal SD card, therefore, only data can be moved to the SD card if it has been allowed by the apk author. To move this data to the SD card, you can go to Settings > Apps and select the app which you want to move to the SD card and press 'Move to SD' If the option is greyed out, it means the author has not allowed 'moving to the SD card'. You can move the data back by pressing 'Move to phone'