1. From Setup:If you’re using Lightning Experience,enter App Manager in the Quick Find box, select App Manager, and then click Edit next to your app.If you’re using Salesforce Classic, click + and then Customize My Tabs.Move Forecasts from Available Tabs to Selected Tabs, and then save your changes.
2. If you’re using Lightning Experience,enter App Manager in the Quick Find box, select App Manager, and then click Edit next to your app.
3. If you’re using Salesforce Classic, click + and then Customize My Tabs.
4. From Setup, enter Profiles in the Quick Find box, and then select Profiles.
5. Select the profile that needs access to forecasts.
6. Click Object Settings and then Forecasts.
7. Click Edit.
8. From the Tab Settings dropdown list, select Default On, and then save your changes.