cannot set cache timeout

Hi,

I installed the premium version and am trying to set the cache expiration. I can insert the hour and minutes, but for the frequency, there is nothing in the drop down list. I also cannot enter anything in the box. If I hit submit, that box turns red, and nothing happens. What should I do?

Thank you for your help,
Dave

https://www.ads-software.com/plugins/wp-fastest-cache/