If you're unable to select a certain jurisdiction while processing a certificate or when configuring tax validations rules, this is because the respective state is not enabled in your company's nexus profile. To address this: 

1. Navigate to your company's Nexus settings by clicking on the gear icon in the app header (you must be a company administrator) and click on the state you want to activate. In this example, we're clicking on California.

2. Once you click on the state, you will be shown a modal to activate it as a Nexus state. 

3. For there on simply activate the status by selecting the drop-down, and hitting the change status button. Once you do so, you can see that the state should now be visible in both the processing and rules areas: