Unable to Edit the Forecast Type of the Opportunity under the Sales Rep Forecast

Context:
1. Navigate to Transactions > Quota/Forecast > Edit Sales Rep Forecast
2. SALES REP = select a sales rep
3. Indicate START DATE/END DATE
4. Under Opportunities, the FORECAST TYPE  is disabled/cannot change value

Explanation:

If the sales rep selected in the forecast is not the primary sales rep of the customer in the opportunity, the FORECAST TYPE will be disabled. This is commonly encountered in Sales Team formats where a group is assigned to customers.