How to read testbot feedback / get more info
If you click the colored patch result (above), you will get to a detailed page. On the top of that page there is a link 'View results on dispatcher'. That will get you to the test. Then go to the 'Console output' (on the left) (direct link).

The actual error appears to be:


  13:56:12 PHP Fatal error:  Class
  'PhpOffice\PhpSpreadsheet\Worksheet\Worksheet' not found in
  /var/www/html/modules/contrib/migrate_spreadsheet/tests/src/Unit/SpreadsheetIteratorTest.php
  on line 187