Mac – How to save password for AppStore on mac？

For years, and it still is true, I've had the pattern that Mail claims to have forgot the password but it's actually reacting to some other problem. If you hit the (Cancel) button on the "enter new password" dialog, you should find that it continues to work just fine.

At one time, this syndrome was clearly associated with network problems talking to the mail server (Microsoft Exchange, as it happened); I could tell this by correlating messages in the Console log. More recently, I haven't always been able to identify the underlying problem, but the trick of canceling the password prompt still seems to work all the time, for me.

You can leave in-app purchases on if you wish.

Once you have enabled Restrictions then at the bottom of the second section, Allowed Content you will see Require Password and underneath they even have a little prompt Require password for purchases.

Change this from the default of 15 minutes to Immediately and your problem is fixed.