To verify, edit the employee record of the user and check if sales rep box is marked. See the detailed steps below.
1. Edit the employee record.
2. Click Human Resources tab.
3. Make sure that the Sales Rep box is enabled. If not, tick the box and click Save.
4. Navigate to Transactions > Quota/Forecast > Edit Sales Rep Forecast. Notice that the user with a sales role can now edit sales rep forecast.
Note: When Team Selling is enabled, please note that only the primary sales rep can edit sales rep forecast. For more details, see the article entitled 'Cannot Edit Sales Rep Forecast to change the Forecast Type of An Opportunity' or search for SuiteAnswers ID 27247.