To enable salespeople to send email campaigns in Partner Center, follow these steps:
1) In Partner Center, go to Administration > Customize > Sales.
2) Turn on "Salespeople can send campaigns".
When a salesperson is trying to start a campaign in the Sales and Success Center and receives the following error: "An error occurred when trying to create this campaign. Refresh the page and try again", it might be an indication permissions have been disabled. Follow the instructions above to enable this setting.