Expansion packs are on sale for a limited time on the buy packs page. These drop as part of a season, and won't be for sale forever. To see which expansion pack is currently on sale you can head to the buy packs page.
Make sure you have funds on Immutable X
Packs are purchased via Immutable X, which means you will need to have ETH in Immutable X in order to purchase, not your MetaMask wallet. If your purchase isn't going through, make sure you have deposited your ETH on Immutable X before trying again. (See: adding funds to Immutable X)
Packs aren't always for sale
Packs are available at specific times during sales, and some packs will never be sold again. If you can't buy packs, you can always purchase individual cards from other users on the Immutable X Marketplace.