the SHIB you have are on the ETH chain. so you can't swap them for BNB tokens, they are separate formats.
do you already own some LUNA tokens, and you want to transfer them to your metamask?
or was your plan to sell the SHIB and use that cash to buy LUNA with BNB, on the new BSC network of your metamask?
if you want to sell SHIB to buy LUNA, that's going to take time and effort.
first you have to swap SHIB for ETH, then you have to swap the ETH for BNB, then you have to use the BNB to buy LUNA.
by the way, if you imported the LUNA contract address into the BSC side of your metamask and it says 0 LUNA, that is normal. your balance of LUNA will only go above zero once you buy the LUNA with BNB on pancakeswap.
