The ability to save cost, sell and yields is controlled by the permission Set Cost & Sell Prices and Yields which is controlled in the role, to review and manage this setting navigate to the follow page:
Click into the Admin Cogs
along the top bar.
Click into Organisation Administration, and into Roles.
Edit the required role, and click into the Permissions tab.
Click into the Advanced Stock section, locate and check the permission Set Cost & Sell Prices and Yields.
Click Update at the bottom to save the change.