If you use nil as a key when declaring an object, an error will occur.

[kimpure]

Examples:

But when I declare it like this, it doesn't appear

[Ketasaja]

Your second example is using the string key "nil". For non-string keys, they must be surrounded with brackets, like [nil] = "true", but nil and NaN are the only two values in Luau that are illegal table keys, so it will error.