For some Saleae devices, the maximum sample rates are only available in certain configurations:
For Logic Pro 8 and Logic Pro 16, the maximum digital sample rate (500 MSPS) can only be selected when 4 or fewer digital channels are active and all analog channels are disabled.
If you are still not able to select 500 MSPS, it could be caused by using a USB 2.0 connection instead of a USB 3.0 connection. Review this article below.