Occasionally, you might try to access an email, but receive a message telling you that you don't have access. There are a couple of reasons why this could have happened:
The email was forwarded by a colleague, who chose to split the email into a new conversation without including you.
A new email conversation was created from the conversation, because your colleague removed a you from their reply.
If you need access to the email, ask your colleague to include you in the conversation.