A Disc Space Error appears when there is not enough free space on your phone for WeddingMix to work. You do need approximately 1gb free so you have room for photos & videos on your phone.

To manage the storage on your device, go to SETTINGS->GENERAL->USAGE->MANAGE STORAGE

By clicking the Edit button you can delete files that are taking up space.

The easiest way to clear up space though is to do the below:

When you delete photos on your iPhone, they don't free up space on your phone immediately. 

To free up space on your iPhone, go to photos> recently deleted folder

Then click "select" in the upper right.

Then "delete all" on the bottom left.