Sorting table: error message

I’m completely new to LibreOffice. I’m trying to sort a 3-column table that was originally created as a Word (.docx) document. It’s now been saved as an .odt file. When I select the entire table and try to sort it by the first column, alphanumerically, I get the message “Cannot sort selection.” Every row has 3 columns. I created another 3-column table from scratch and was able to sort it successfully. Why can’t I sort my old table?

It would be helpful if the error messages explained WHY something can’t be done, so the problem could be fixed.