Hello,
Please do not use HTML builder, its time taking and an obsolete one. make sure that you are using theme / plugins of version 3.9.6
please try to use pdf certificate builder because it's great.
delete your current version of pdf certificate plugin or update this plugin from plugins section.
or download from here and then upload zip file: wplms-pdf-certificates
take full page width of the element then use center align.
tutorials:
1) https://www.youtube.com/watch?v=d9mXInbmFGY
2) https://www.youtube.com/watch?v=ylT1j35dUbg
3) https://www.youtube.com/watch?v=kyJGVlegWTc
manually assign certificate:
http://somup.com/cqQYIreCJT