As Kraynic said, you should be able to add weight to any item on the character sheet. If you're not able to, then check if you have any browser extensions running, and try opening Roll20 in a private/incognito browsing window to see if that resolves the issue. I'm not sure if you are the GM, but if not then also check if the game is using any API scripts (the game creator must have a Pro subscription) that might be affecting item weight inputs.