Found the problem, and I believe that the issue can be firmly placed on Microsoft. The Store installer has a problem using particular hard drives. When attempting to install on:
Samsung SSD 850 PRO 1TB – Download fails in 2 seconds (before even checking the license) and installation stops (code: 0x80073CF9).
WDC WD1002FAEX=009A0 (Western Digital) – Download fails in 3 seconds and retries immediately (3 times) before stopping (Speed related?).
Fantom External HHD USB Device – Installs without a problem.
All the above drives are on the same computer. The problem may be that the installer is confused by 3^rd^ generation SATA (6Gb/s), as it installs on almost any USB connected drive. However, my Surface Pro has an SSD with 6Gb/s SATA and it installed without a hitch.
**Solution:**All those numerous “fixes” proposed by Microsoft may be useless for 0x80073CF9 errors. If the fixes do not work then plug in a USB drive and, when prompted, select the USB and see if it works.
Conclusion: I am extremely disappointed that Microsoft leaves the users to find the glitches in the coding. If I can find where the problem is in 2 days, then what are the MS testers being paid for; and why is there no feedback to the Age of Empire: Definitive Edition users that have 0x80073CF9 problems (I can picture the SQL to get the list of such users)?