If you won a gym battle and did not get a badge, work through these checks.
Check the Common Causes
1. Was this your first win against this leader? Badges drop only after your very first victory against each specific leader. If you beat them before, even long ago, a rematch will not drop another badge.
2. Did you look on the ground? The badge drops as an item. It does not appear in your inventory automatically. Look around the spot where the battle ended.
3. Did it despawn? Dropped items in Minecraft disappear if they sit on the ground for several minutes. Unloaded chunks can also cause trouble. Pick up the badge right after the fight.
4. Was it really a gym leader? Only gym leaders from Radical Cobblemon Trainers drop badges. Ordinary trainers do not.
Check the Setup
1. Both mods must be installed. The data pack links the loot of Radical Cobblemon Trainers with PKGBadges. If PKGBadges is missing, there is no badge item to drop.
2. The data pack must be enabled in this world. Run /datapack list and confirm the pack appears in the enabled list.
3. On a server, the data pack has to be in the server's world. Adding it to your own single-player save does not affect the server.
If all of these check out, beat a leader you have never fought before to test it.