Your most recent achievement may be finishing uni. But imagine how many other people this is the same for! Using uni as your greatest achievement won’t make you stand out from the crowd.
Try to consider an alternate achievement such as, completing a marathon, organising a charity event, volunteering or learning another language. Relate these to the skills the job role may require such as perseverance, working on your own initiative or teamwork.
If you are struggling to come up with an achievement then sometimes mentioning that moving away from home to university (if you did) was a huge achievement and it helped you to become more independent in managing your time