Okay. The p721.dat is not your concern, it is created in C:\Users\<yourusername>\AppData\Roaming\CaptainSim\p721 folder once you successfully Activate the 727-100 Base Pack.

1. Uninstall the 727-100 Base Pack
2. Delete any Code_p721.txt, and Key_p721.txt files you find, and delete the p721.dat file.
3. Turn your AV, and Windows Defender OFF (if you don't one of them might delete some important files during the installation, which will prevent the 727 from working).
4. Install the 727-100 Base Pack, use Manual Offline Activation, and when it asks, select GENERATE AND SEND YOUR SYSTEM CODE, then close the installer.
5. Save the generated Code_p721.txt to you Desktop.
6. Go to this website: https://www.captainsim.com/activ/offline.php, and Browse to your Desktop, and select the Code_p721.txt, then select Get a Key.
7. Save the Key_p721.txt to your desktop!
8. Run the 727-100 Base Pack installer, select Manual Offline Activation, and when it asks select USE A KEY RECIEVED FROM CAPTAIN SIM, browse to the Desktop and select the Key_p721.txt you just saved there, and when it gets accepted, complete the install.
9. Turn your AV, and Windows Defender back ON.

Repeat the above for the 727-200, and 727 Freighter if you have any of them.
The files are Code_p722.txt, and Key_p722 for the 727-200 Expansion Model, and Code_723.txt, and Key_p723.txt for the 727 Freighter Expansion Model (and delete the p722.dat, and p721.dat, which are found in C:\Users\<yourusername>\AppData\Roaming\CaptainSim\p722/p723 folders) 

Start FSX, load a CS 727-100 and everything should work.