How to fix error on site load

hI! i'm new to Plone but when i try to log on i get the error
AttributeError("'RequestContainer' object has no attribute 'portal_factory'",) (Also, the following error occurred while attempting to render the standard error message, please see the event log for full details: 'NoneType' object has no attribute 'queryMethodID')

Hi and welcome!
Could you please provide us with your versions of Zope/Plone and which steps you took exactly that led to the error?

I use Plone 5.1
i installed one of the available add-ons not sure which but is one of the listed ones, then that happened

It would be good to know which one, because at the moment I can't reproduce the Error.

Provide the full traceback

In case someone lands here from Google, I had the same 'NoneType' object has no attribute 'queryMethodID' error and in my case I was able to understand where it came from. Maybe a different problem from OP, but might still be useful to other people.

I was removing Archetypes from a website and following @fulv recommendation. When doing so I saw a fulluninstall profile and used that instead of the recommended one. This caused the error: the call to queryUtility(ITypesTool).getTypeInfo(self) was often returning None for dexterity content types.

To solve the problem I restored an older version of the ZODB and ran the correct uninstall profile.