@D_Lasa 
it’s hard to tell because the error messages you shared aren’t formatted very well, but it looks like all the error messages were referring to the now-deprecated repositories. These were replaced last year with a temporary repository, as detailed in the first blog you linked to:
I would encourage you to review that blog post as the information there is still valid (though not for long); specifically including the new repository DNS record: eol-repositories.sensuapp.org
The packages you need to upgrade to 1.9.0 will still available, but only until Monday (Feb 1): Index of /apt/pool/xenial/main/s/sensu/
I hope this helps!