How to run SQL services on NT SERVICE\MSSQLSERVER account if it is running earlier on LocalSystem
You have to change the service account in SQL Server Configuration Manager in order to have all required permissions applied to the service account.

Open Configuration Manager, select SQL Server Services in the treeview on the left, then open the properties window of the desired SQL Server instance.

Set the service account as follows:



Then open file explorer, navigate to the SQL Server data root and change the permissions on the properties window. Click the Advanced button:



Click the "Change permissions" button:

 

Then add the service account using the add button. If your computer is in a domain, make sure you select the local computer and enter NT SERVICE\MSSQLSERVER:



Make sure you check the "replace permissions on child objects" checkbox.