Can we use a scale even if one of the answer levels was not added?
I think you'll come unstuck if you don't have a balanced scale.

If you consider a typical five-point scale:


Strongly disagree
Disagree
Neither agree nor disagree
Agree
Strongly agree


If we forget to put Strongly disagree on the scale, it is positively biased, because it is heavier on the Agree side of the scale. If we forget to add Strongly agree we have a negatively biased scale.

If you intend the questionnaire to be used for research purposes, you'll need to ensure the scale is correct - otherwise it may affect your findings, and it will definitely cause people to question whether the outcome would have been the same as if the correct scale was used.