1st Possible Reason
If your report doesn't have styles try to add some of these parameters
in Settings > Parameters > System parameters:
(first open the website in debug mode then go to setting and in technical
you will find parameters and in side it system parameters)

web.base.url: :

web.base.url.freeze: True. If this parameter is not used then if the 
administrator log in Odoo the web.base.url parameter is updated with
the current addres that's in the address bar.

report.url: :. This should be used if web.base.url
parameter is not enough to make it work. Usually this url should be: 
http://127.0.0.1:8069


2nd Possible Reason
Also it is possible that the package wkhtmltopdf is not well installed 
or it has not the correct version. Here you can check the release 0.12.5
that works well with Odoo v10 and later versions.

3rd Possible Reason
As it is suggested here, the report contents must be inside this 
element with that specific class: