Is salt required in Argon2
If you use a fixed salt all your passwords can be attacked concurrently.
Though argon2 is designed to be slow and thus hard to do a dictionary attack I see no reason to give your attacker a speedup factor of the number of users.
This will obviously be on top of any other advantages the attacker may have(e.g superior hardware or cryptographical advancement).
On a system with 100 users, using a salt gives you a 100x advantage over an
attacker doing a dictionary attack on the hashed password. You can use this to slow her down or alternatively tune the security parameter to make your legitimate hashing faster and less resource intensive.
Using a salt is an easy win.