After Upgrading my XWiki to 11.6. When I create a topic I get this error message, why is that? 
This error suggest that a Converter does not implement any of convertToType(Class<G> type, Object value) or convertToType(Type targetType, Object value) (which are calling each other for retro compatibility reasons) which does not make much sense for a Converter.
Would be interesting that you share the complete error to see where this is coming from.
After Upgrading my XWiki to 11.6
It’s not recommended if you just did that. The current stable (and soon LTS) version is 11.10.