Hello there @Supreme_Goose,
Can you go onto NameMC and search for the player's name there, reminder that we only support accounts bought from Minecraft's official website.
If there are no accounts showing with the name, then it's a cracked account and we don't support it, however if there are accounts showing with the name then it's a temp issue with Mojang.
If your sure that you are trying to whitelist a non-cracked player, try unwhitelisting, get the player logged in, rewhitelist then add them to the whitelist.
If this issue stills occurs, I would suggest contacting support here and clicking "Contact Support" on the page linked.