First, When you walking, your two legs still moving, there is no a moment your two legs both stand steady, so this Frame will break your animation.
Second, Walking animation is a loop, I create a very simple sprit sheet:
Step 1. Right leg stand steady, Left leg raise
Step 2. Right leg move backward a litte bit, Left leg move forward
Step 3-4. Right leg copy the actions of Left leg, Left leg copy the actions of Right leg.
The legs just need raise-move-stand, then raise-move-stand again.
It’s just need 4 frams, then you can add more additional frams between them, let it became 8 frams, 16 frams or more.
Just remember it’s a loop, some frams are same, just another leg use the same action.
And this one you create before:
Still make the same mistake, a two legs both stand steady frams
I just need delete this one frame, then your walking animation became this:
Did you see the difference?