If you are on the Starter plan you can't collaborate on documents - you need to upgrade to our Premium plan. You can still share documents you have created yourself through a web shareable link but they will be static web documents with no collaboration enabled.
Additionally - if a colleague attempts to share a document with you and they are on a Starter plan you will not be able to collaborate on the document either - you must both be on the Premium plan.
Find details of our pricing plans here.
If you can't get open a file that someone has shared with you it might be because the email address in the link that they sent you doesn't match your Xara Cloud sign in details. If it doesn't you will receive this error message when opening the link.
You will need to add or update your email details to match the one in the link.
See this article for info on changing or updating emails and passwords.
For more on document sharing and sharing editing rights have a look at these articles...