Is it possible to just not have an acl?

Yes: Agenesis or congenital absence of the ACL has been described in patients who are born without an acl. It is rare and may be associated with absence of a meniscus, limb length discrepancy. Typically the patients have symptomatic instability causing them to seek care, which usually requires surgical ACL reconstruction.

Rare: It is possible but this would be a congenital abnormality and very rare.

Extremely rare: Very few cases have ever been identified of patients born without an ACL. It is more common to have an ACL removed after a traumatic incident - some of these patients have continued playing competitive sports with ACL focusing on strengthening the other supporting muscles and ligaments instead to stabilise the knee