There are 3 warnings or indicators at the bottom of the template. These will tell you the DPI of a selected layer, whether or not all layers meet the DPI requirements and if the template is covered (green checkmark). If a layer is not of the correct DPI or higher, or the template is not covered, you will not be able to save your design (red x). The save button will be grey instead of blue, and you will not be able to select it.
Ready to save:
Not ready to save: