Grow connects to Salesforce via an API connection. Salesforce only offers the API and functionality needed on their Enterprise level tier.
In order to access Salesforce in Grow, your organization must be on the Enterprise tier in Salesforce.
If you have the Professional Edition and need API access, you will need to contact your Salesforce Account Executive to log a case with Salesforce support. They will enable it for an additional cost.
You can find more information on the Salesforce Pricing Page.
Questions? Email us at firstname.lastname@example.org or chat in on the site.