Hi Nazario,
You've got three main places to look if your watch isn't charging; the charger, the charging cord and the battery. Odds are it's one of those three things in most cases.
You'll want to try a different charger and/or a different charging cord to try and eliminate those as a suspect. If it still doesn't work, the next most likely source of a failure to charge would be the battery. Fortunately, iFixit has both a guide to replacing the battery available as well as selling the battery itself, so if that's the problem, they've got you covered. Here's a link to the guide you'll need to do the replacement.
[guide|143163|Samsung Galaxy Watch Active2 Battery Replacement - iFixit Repair Guide|new_window=true]
You'll need to order the correct battery for your watch; it appears they come in 40 and 44 mm models so pick the one that fits your watch. The guide has links to both where you can purchase them.