Have you added the polygon network? https://metamask.zendesk.com/hc/en-us/articles/360043227612-How-to-add-a-custom-network-RPC
You’ll also have to add any tokens you wish to interact with through Metamask.
How to View Your Custom Tokens in MetaMask
You can view your wallet address on a blockchain explorer (https://polygonscan.com/) to see what tokens are in it.
Personally, I use the CoinGecko method from the link above (I always mess up somehow when manually copy/pasting )
If you enter something incorrectly, it wont break anything. That only affects how tokens are displayed. For example, if you put “17” in the decimals place (instead of 18), that tokens value would show as 10x higher than it should. You can always just remove a token that isnt displaying correctly and add it again.
Helps to think of Metamask as a way to view and access the accounts that are on the blockchain, not the actual account itself.