Learn Dutch: Deze nieuwe collega is helemaal niet creatief. - This new colleague is not creative at all.

Adjectives borrowed from French or Latin, especially those ending in '-ief', take the suffix '-iteit' rather than '-heid'. When adding it to creatief, the 'f' changes to a 'v', resulting in the noun creativiteit.

Could I say creatiefheid instead, since '-heid' also makes nouns out of adjectives?

No, creatiefheid is incorrect. The suffix '-heid' is generally reserved for native Germanic adjectives (like snelheid for speed or gezondheid for health). Because creatief is a Romance loanword, it strictly takes '-iteit'.

How would the sentence change if I used the noun creativiteit instead of the adjective creatief?

You would need to change the verb and the type of negation. Because you are now negating a noun rather than an adjective, niet becomes geen. The sentence would be: Deze nieuwe collega heeft helemaal geen creativiteit. (This new colleague has no creativity at all).

Why does nieuwe have an '-e' at the end, but creatief does not?

Nieuwe acts as an attributive adjective because it comes directly before the noun collega, so it gets the standard '-e' ending. Creatief is a predicative adjective coming after the verb is, so it stays in its basic, uninflected form.