YOur code didnot through any validation errors but it has warning of the line you specified because you have choose to use two columns in the header and not followed in some rows like this for easy elaboration.
YOu have started with following
<table>
<thead>
<tr>
<th>First name</th>
<th>Last name</th>
</tr>
</thead>
table with two columns of header and in the first row followed header you have start row with only single column like this.
<tr>
<td></td>
</tr>
you need to replace this with
<tr>
<td></td>
<td></td>
</tr>
and in the last row you repeat the same mistake by creating row with single column