The Actual Start and Actual Finish fields are not allowed to modify them.
In our application you cannot change the Actual Start Date, but as a workaround, after you created your plan, please set a baseline in order to see a difference between Planned Start Date and Actual Start Date.
In this way, the Scheduled Start date is in fact the Baseline Date.
After this, if your task actually starts on other date you can change the Start Date for that task. This will be the Actual Date.
So, the Actual Date is in fact is the Start Date and the Scheduled Date is the Baseline Date you set in the application when you initially created your plan.