The "Main/Welcome" web does not exist
is divided into webs; each one represents one subject, one area of collaboration. You are trying to 'view' in a web that does not exist.
If you came here by clicking on a question mark link
A link to a topic located in another web is written like Otherweb.TopicName. Make sure that the name of the web is not spelt wrongly on the previous page; fix the link if necessary.
If you would like to create this web
You can create a new web
if you have permission. Contact firstname.lastname@example.org
if you have any questions.