I have a second gen MOTO -E . I have 220 MB free in internal storage but I can't download apps of even 3 MB What should I do ?
To be able to download apps in the Play Store you will need at least 510mb available or 10% of the internal memory. Also depends on the size of the app you download. The Android OS uses the internal memory also to RAM purposes so is very relative the available space. Try to clear cache and data from GPStore, GPServices, Reset Apps Defaults and also remove some unused apps.
Also check the link provided by bmdixon