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I click"CONFIGURE" then click on the "Block Select Mode" box.

I highlight ONE character(s) in the top row of the desired column then click "SEARCH", click on "GO TO", click on the "LINE" box, type +99999, click on "EXTEND SELECTION" and click "GO TO".

Instead of highlighting ONE character in the desired column, 15 characters are being highlighted.

What am I doing wrong?

I tried your instructions on a text file and it worked fine for me TP 5.0.3.

In configure -> preferences > editor, I have "constrain cursor to the text" and use MS Application keystroke compatibility.

I would use a quicker method to achieve what you are doing, I would:
to get in and out of "Block Mode" just double click on "Block" on the status bar.
to select a column I would put the cursor on the desired column, hold the shift key down and do a right arrow to select the character, then with the shift key still down press the Ctrl key + End. That should select the column.

Give it a try, if nothing else it might eliminate some possibilities.

In configure -> preferences > editor, "constrain cursor to the text" was checked but MS Application keystroke compatibility was not so I checked it and my problem has been resolved.

the key stroke compatibility should not really make a great difference.

Some times it is how the last line of the file terminated (with a carriage return or not).

Anyway it is working - good.