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.