What is wrong with my phone’s memory?

Hello! I am not able to download anything on my phone from Google Play Store. It gives an error and says "Insufficient memory in memory card". How is this possible? I only have WhatsApp installed in my phone, no other app. The rest of the things are there by default like Google Maps, Google+, YouTube etc. This shouldn't be happening as the memory card has the storage capacity of 1 GB. What is the reason? Thanks in advance!
