HubSpot-Salesforce integration is not currently installed
Install the HubSpot-Salesforce integration. To connect your Salesforce sandbox to HubSpot, ensure you select the Yes, this will connect to a sandbox checkbox, and install the integration package in your Salesforce sandbox.
HubSpot-Salesforce integration is currently installed
If the integration is already installed, but your Salesforce production organization is currently connected to HubSpot, connect your Salesforce sandbox to HubSpot: