Why my mobile shows that device have not enough space

[AC Question]

My mobile have 16 GB memory card

 

[ManiacJoe]

Your phone has internal storage of a fixed size.
If your phone accepts micro SD cards, this external storage also has a fixed size.
The SD card is a good place to store your pictures, videos, and documents.
Your apps get installed to the internal storage. Most of the time they cannot be moved to the SD card.
If your phone is giving you a message about low storage, it is probably referring to your internal storage.