Aetna denying claim

I moved here from the UK a couple of months ago, so I’m still very much learning how things work.

I started a job on 09/01 which offers health insurance – I enrolled in a PPO plan with Aetna through my employer on 09/01 and in my HR site it states coverage begins 09/01 and deductible period begins 09/01 (I can’t seem to login to Aetna – every time it just says unsuccessful login, try again later, despite having the correct username/password, trying different browsers, clearing history and cache etc.).

I had an appointment with an in-network PCP on 09/09 to get prescriptions sorted out (I have type 1 diabetes, so obviously this is very important). I received a letter today saying the claim cannot be processed as I was not eligible for benefits for the submitted dates of service (earliest date of service is listed as 09/09).

Am I missing something? Is this just an error? Or is there typically like a grace period or something I’m not aware of? I plan to phone them tomorrow, but I just wanted to see if I was missing something stupid prior to that conversation.