You are attempting to clone a form by selecting More Actions > Clone and receiving a "You must select an existing form" error:
by clicking the New Form button and selecting to Clone the form and receive the error "List has no rows for assignment to SObject" :
You've changed an object's Name field from Text to Auto-Number, and are attempting to clone a form where the Name field is mapped to a text field, but isn't identified as the ID for the object. You cannot map an auto-number field that is not also entered as the ID Field for the object.
In order to be able to clone the form, change the Name field back to a Text field, clone the form, and then change the Name once again to an Auto-Number field.