Look at the job deskription or role requirement. Select one that you are not comfortable with or comparably weaker than the rest of the requirements. That is what your weakness is relevant to the position you are applying for. After pointing your weakness, make sure what you are doing to be better at it and what are the plans ahead for the same. If it is related to tool, find its similarities with something you are better at , pointing the similarities, showing them you can learn it pretty quickly.