Welcome to the community, @Jimmy_Mitchell!
One of the most annoying & frustrating things about Airtable formulas — which, for some reason, the Airtable engineers have yet to fix — is that Airtable formulas choke on curly quotes. You need to replace any curly quotes with straight quotes only.
Although, in your case, it doesn’t even look like you’re using curly quotes. It looks like you might have used a backtick, which is the wrong symbol.
Put the word “days” in single, straight quotes like this: