1st Possible Reason
If your report doesn't have styles try to add some of these parameters in Settings > Parameters > System parameters:
web.base.url
: <IP address>:<Port>
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
: <IP address>:<Port>
. 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: <div class="page">
report.url
or theweb.base.url
system parameters)? – Audiophile