OK, I solved my problem. Thank you to the moderators for their quick offers of help.
I had copied a running blog on a remote server to my local machine in order to be able to check if 2.3 will run OK with my theme.
I backed up the mysql data from the remote server using the backup plugin. I opened phpmyadmin, created a new database and imported the data from the backup. So far so good.
However, I had not set the permissions for this new database. I was already a user with password but I had to make this user/password combination work with the new database I had created.
For others who may have the same problem, here's how:
In phpMyAdmin, from the Home page, I clicked on 'privileges'.
I found my username in the list of users and clicked on the pencil icon to edit the privileges.
In the first block of possible privileges-global privileges- I selected all then deselected 'GRANT' and clicked 'GO'
Then in the section below-database specific privileges-I selected the database I wanted access to from the dropdown menu and clicked 'GO'.
And it worked!
So, yes, this is an example of one of the 99.9% human-errors and moshu's post did give me the clue to what might be the problem--but not how to solve it.
To work this out I used Brandon Booth's installation instructions (www.lucidgreen.net) and Chapter 2 'Creating a local test environment for your blog' in the book Blog Design Solutions published by Friends of ED.