In order to create a stack/ePoster in Learning Toolbox for a conference you must have activated the invite link you were sent for that conference stack/ePoster and you must have signed in using the same account you used when you activated your invite link. If you have not received and activated your invite link then read this FAQ answer [How do I find my invite link].
Do you see the Getting Started Page (image below) when you click on Create Stack?
If you see the Getting Started Page then this means your account does not have permission to make an ePoster/stack. There are two possible reasons for this:
You are not signed in. Check that you are signed in. If you are not signed in then click on Sign In and make sure to sign in using the same account as you used when you activated your invite.
You are signed in but not with the account that you used when you activated your invite. Please sign out and then sign back in again making sure you use the same account that you used when you activated your invite. You will have received an email from Learning Toolbox (firstname.lastname@example.org) when you activated your invite reminding you what account you used to do this.