I've taken a look at your project.
Since you're working with Spine Essential you do not have the benefit of using an IK Constraint to pin the feet to the ground. So to make it appear as if the feet are not sliding on the ground you can use Ghosting
to view frames before and after and adjust the position of your bones. You will never get completely rid of sliding, but by using ghosting you can limit it to a degree where it's not noticeable.
Please see Ghosting - Spine User Guide for more information on how to use ghosting.