Correct spelling of fldCharType separate. See http://www.ecma-international.org/news/TC45_current_work/OfficeOpenXML-XMLSchema.zip , file wml.xsd, line 3977.

Author: pjt33

src/PhpWord/Writer/Word2007/Element/CheckBox.php

@@ -68,7 +68,7 @@ public function write()
         $xmlWriter->endElement(); // w:r
         $xmlWriter->startElement('w:r');
         $xmlWriter->startElement('w:fldChar');
-        $xmlWriter->writeAttribute('w:fldCharType', 'seperate');
+        $xmlWriter->writeAttribute('w:fldCharType', 'separate');
         $xmlWriter->endElement();// w:fldChar
         $xmlWriter->endElement(); // w:r
         $xmlWriter->startElement('w:r');