Pixel 6a comes from factory with Android 12, but after you install the updates it ends up with Android 13. You need to update the phone first to be able to unlock it:
For the Pixel 6a, OEM unlocking won't work with the version of the stock OS from the factory. You need to update it to the June 2022 release or later via an over-the-air update. After you've updated it you'll also need to factory reset the device to fix OEM unlocking.
After you update the phone using stock OS, you'll be able to do the OEM unlocking and proceed with the installation of GrapheneOS.
It's all very simple, and takes only a few extra minutes.
Here's some blog post about it: https://miguelr.net/posts/pixel6a-grapheneos/
And here's the official installation: https://grapheneos.org/install/
At the end of the installation you'll end up with GrapheneOS based on Android 13. You'll also receive updates for 5 years.