Problem installing 3-way TP-Link HS210
There are two travelers on each of the existing switches, no neutrals. If there's nothing else in the single box, that common wire goes to the fixture. The second box is where your feed comes in. Don't mix up your white traveler and connect it to a white neutral. You will probably end up with two white wires going to the switch in the second box, one traveler and probably one neutral... don't mix them up or connect them together... tape the travelers at both locations with some yellow tape to prevent confusion in the future.
Don't all it a day with an unconnected ground, You need to extend it to the switch.