I download the file, extract to the wow classic interface folder and it doesnt show up, not sure what else to do, no other addon is having this issue

Figured it out, had to rename the extracted folder, was "WeaponSwingTimerAddon-master" showed up once it was set to "WeaponSwingTimer"

Yeah, I wish Blizzard would let you know that it couldn't load an addon because if illegal characters. It doesn't like '-' as you've found out.