Setup a visible when condition for the left side panel. The when condition should key off the requestor data from your external user entering the Pega Platform. Find what access group the external user is using when entering the application, key off that value to hide the left side panel.
If multiple different types of operators are using the same access group, either use a different value that be found off the requestor clipboard page that is unique to your external users or create a separate access group that is specific to the users using the email link to get into the assignment.
pyshowFullPortal attribute value is already changed to True.While clicking the link for the first time, it is opening properly. Due to SSO, once closed the browser and opening the link again, facing this issue.