Hello! It works without errors! Thanks for all the help!
In short, the error, it seems to me, was largely due to the cache:
Uncaught (in promise) TypeError: Could not execute 'addAll' on 'Cache': Request failed. this error message persisted to the end.
There were several steps and it is difficult to give credit to anyone.
The solution to the "No apps available to update" problem was a collective contribution that I'll try to summarize:
[Randy Calvert]
1) Log in to safe mode after uploading a php file called "constants.php" containing the line of code:
<?php
define('RECOVERY_MODE',True);
2) Once logged in, remove the line of code or the entire "constant.php" file from the server.
I think at this point it would have been better to have continued with:
[Randy Calvert]
Use the support link and then click the link to clear system cache.
but I continued with:
[Jim M.]
1) Disable URL rewrites in ACP > System > Search Engine Optimization.
2) Download ".htaccess" provided on the same page and upload it to the server.
3) Enable URL Rewrites in ACP > System > Search Engine Optimization.
arrowman's post in "There are no apps available to upgrade" was marked as the answer