Your administrator is correct. It's a reason many plans don't allow after-tax (nonRoth) contributions. Aftertax contributions are usually only made by HCEs (as HCEs are usually the only ones with the discretionary income), so it can be difficult to pass the ACP test with them. Also, after-tax isn't exempt from testing, even in a safe harbor plan.
Not only that, but I think the plan would also lose the Top Heavy exemption (if it were otherwise top heavy) if after-tax contributions were made on top of deferrals and match.