Try one of these or the link at the bottom.
Method 1: You may try running it with administrator privileges.
a. Right-click on the game’s shortcut and select Run as administrator.
Method 2: Try giving permissions to the folder which contains fsx.cfg file.
a. Locate fsx.cfg file. The path to this file is given below.
b. Right-click on FSX folder and select Properties.
c. Click the Security tab.
d. Click Advanced, and then click the Owner tab.
e. Click Edit, and then do one of the following:
• To change the owner to a user or group that is not listed, click Other users and groups and, in Enter the object name to select (examples), type the name of the user or group, and then click OK.
• To change the owner to a user or group that is listed, in the Change owner to box, click the new owner.
f. (Optional) To change the owner of all subcontainers and objects within the tree, select the Replace owner on subcontainers and objectscheck box.
Refer the link below for more information:
Take Ownership of a File or Folder