Thank you. The problem is solved. I opened your link and found the section "Uninstall Microsoft 365 for your installation type". I followed it and uninstalled Office in
Settings->Apps. Then I used the installation instructions supplied by the seller and everything worked fine.
The astonishing conclusion is that if you start (but never finish) installing Office on a PC, Microsoft will remember it even after a complete system reset, and will not let you install Office once more.