Some of the coins such as ethereum, tron, stellar... have no limit to the production of tokens and therefore do not have a max-supply.
This implies that, since the "If IT was Bitcoin" value is calculated from the ratio between the max-supply of Bitcoins and the max-supply of the altcoins, if an altcoin does not have a max-supply, this ratio would indicate an infinite result.
It is for this reason that coins without a max-supply cannot be analyzed by this website.