Unable to change step due to portal user

Overview

When trying to change step, you may receive an error saying:

"Cannot be set to closed: There is 1 portal user which would have no Matters or will create a task while being closed"

Solution:

Go to the Matter, click the Portal tab and see if there are still users who are ticked under "Portal Access". If so, untick them. After this, you will be able to change step.