Table columns in range X…Y established by element “th” have no cells beginning in them.

A <th> with a colspan wider than the rest of the table adds the columns in the reported range, and no <td> or <th> begins in any of them.

The validator builds the table grid from each row's cells and their colspan values. A <th colspan="4"> that starts in column 2 covers columns 2 to 5, so the table has at least five columns. The HTML table model requires every column to have at least one cell that starts in it, and a cell that only crosses a column through colspan doesn't count. If the other rows have only two cells, nothing starts in columns 3 to 5, and the validator reports them as the range 3…5, naming th as the element that created them.

This often happens with header rows that group headings over more columns than the body has, or with a colspan left over from a wider version of the table. The short rows usually add a warning too: "A table row was 2 columns wide, which is less than the column count established by the first row (5)." When only one column is empty, the validator names that single column ("Table column 3") instead of a range.

To fix it, lower the colspan so the header covers only the columns the rows use. If the table really needs the extra columns, add cells to the other rows so a cell starts in each of them.

Invalid example

The second heading spans four columns, but each body row has only two cells:

<table>
  <tr>
    <th>Term</th>
    <th colspan="4">Dates</th>
  </tr>
  <tr>
    <td>Autumn</td>
    <td>September to December</td>
  </tr>
</table>

Valid example

Without the colspan, the header and the body both have two columns:

<table>
  <tr>
    <th>Term</th>
    <th>Dates</th>
  </tr>
  <tr>
    <td>Autumn</td>
    <td>September to December</td>
  </tr>
</table>

Learn more:

MDN Reference

MDN Web Docs: The th element

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MDN Reference

MDN Web Docs: The table element

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WHATWG Spec

HTML Living Standard: Tabular data

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