Hi Claire, we had the same issue in the past, and by this feedback from Salesforce support we were able to solve it:

“Solution: Usually if you cannot select an admin for the Pardot sandbox it is because the sandbox is old (created before you purchased a Pardot sandbox) and the licenses are not up to date.
That is fixed by matching licenses. Can you please go ahead and follow the steps here to refresh licenses in your Salesforce sandbox and try again to create a Pardot Sandbox after that? https://help.salesforce.com/s/articleView?id=000335272&type=1 “

Hi Claire – Unfortunately, this isn’t something that we have experienced. Maybe you can submit a ticket to Salesforce Support for help?

Salesforce’s help docs have a nice step-by-step walk through for the new Sandbox features. Keep in mind you’ll need Advanced or Premium edition of Pardot for that.

If you’re using a lower edition, your best option is to request a Pardot training environment as in item 9 above. When you have that, you can add a new Salesforce connector in that Pardot training account. You should see a “Sandbox” setting to enable, and then you’ll supply the appropriate credentials.

Is it still true that a Pardot Sandbox can only be provisioned from a Salesforce Full Sandbox?
The release notes for Summer ’20 explicitly says “Any Salesforce sandbox”. So I would expect that it also works in developer sandboxes, no?

“This sandbox is a Pardot Business Unit that is provisioned from any Salesforce sandbox.”
https://releasenotes.docs.salesforce.com/en-us/summer20/release-notes/rn_sales_pardot_sandbox_ga.htm

Thank you!

If you can’t push Pardot sandbox changes to production and you can’t replicate what you have on production to sandbox either (and you have to create manually all the processes), it’s hard to me to understand the benefits of the new Sandbox.

I use the training environment and I connect the Sandbox Salesforce environment to it and it will allow me to do more or less the same. Am I missing someting?

Cheers!