On the state parks website it is stated that some state park offices are closed due to covid. I will be camping soon and need a day pass, will I be able to purchase at the park? or do I have to purchase online?

The office is open.

The station was open when we went last weekend.

You can get day passes at the park entrance. There's a box with slips to fill out, envelopes to fill with cash/check and drop in. I believe the station is usually staffed as well for credit card payments but can't guarantee depending on your time of entrance.