The following did not work well. When I find a solution, I will update this post.
When I removed a column from a table, ACCESS gave me a message saying I first needed to remove the link(s) from the relationship window.
This means that a column I wanted to delete had an existing relationship (or relationshipS) with another table(s). The following is the summary of what I did.
I have to break these link(s) first. The problem is when there are a lot of tables in the relationship window, relevant tables and links between them are hard to locate.
I suspect in some cases some tables are hidden for an unknown reason.
Pull the table away from other tables, so you can clearly see the links themselves. Click each one of them to see with which table the table is linked. You will eventually find the table you are looking for. Remove the links to break the relationships. Then you can delete the columns you want to delete.
Up to this point, I thought the problem was solved, but I keep getting the message, "Enter Parameter Value." "Birth month" as it appears in the message graphic is one of the two variables/columns I deleted. I got stuck.
I found this video, which may or may not be relevant to solve my problem.