Metamask is not natively BTC compatible. By default we work on Ethereum Virtual Machine compatible chains (Ethereum mainnet, BNB smart chain, L2 solutions like Polygon, etc)
The flasks feature enables support of other networks, but it’s an experimental feature, and not one I would recommend diving into for most users, just yet.
Alternative option would be to hold wrapped BTC. Its an ERC20 token that is pegged to BTC.