Your Harvest account can only be connected to one Forecast account. If someone else on your team has connected your Harvest account to a different Forecast account, you have two options to resolve this: 

1. Ask your teammate to invite you into their Forecast account. You will then be using a Forecast account that is already connected to Harvest, and you can try Forecast together as a team. 
2. Disconnect Harvest from that instance of Forecast so you can connect your Forecast account. To do so, simply log into Harvest, click on Settings (upper right corner), navigate to the Integrations section and click Disconnect next to the Forecast connection that’s listed. This allows you to then connect Harvest to your Forecast account.