You missing </td> at <td>-</td>

td {
    width: 75px;
    height: 50px;
    background-color: rgb(80, 255, 109);
    text-align: center;
    font-size: 2rem;
}

<table>

  <tr>
     <td colspan="2">AC</td>
     <td>Del</td>
     <td>+</td>
  </tr>

  <tr>
     <td>1</td>
     <td>2</td>
     <td>3</td>
     <td>-</td>
  <tr>
</table>

You are missing a closing </td> tag before you close the table. Try this:

td {
    width: 75px;
    height: 50px;
    background-color: rgb(80, 255, 109);
    text-align: center;
    font-size: 2rem;
}

<table>

  <tr>
     <td colspan="2">AC</td>
     <td>Del</td>
     <td>+</td>
  </tr>

  <tr>
     <td>1</td>
     <td>2</td>
     <td>3</td>
     <td>-</td>
</tr>
</table>

This is because you are not closing your second tr tag

<table>
  <tr>
     <td colspan="2">AC</td>
     <td>Del</td>
     <td>+</td>
  </tr>
  <tr>
     <td>1</td>
     <td>2</td>
     <td>3</td>
     <td>-<td>
  </tr>
</table>

td {
    width: 75px;
    height: 50px;
    background-color: rgb(80, 255, 109);
    text-align: center;
    font-size: 2rem;
}

<table>

  <tr>
     <td colspan="2">AC</td>
     <td>Del</td>
     <td>+</td>
  </tr>

  <tr>
     <td>1</td>
     <td>2</td>
     <td>3</td>
     <td>-<td>

I get an extra box here

</table>

When I run this. I get an extra empty box on the end of the second tr.