How do I improve the Quality Score?

QS doesn't change overnight. It takes time money and traffic ultimately to improve QS (once you have optimized the KW with pages). Blank QS are just Google's way of saying there isn't enough traffic yet to determine your QS. They used to just assign QS 6 out of the gate for all KWs, now they show blanks until a QS can be determined.

Improve CTR. Increase bids. Optimize the landing page. Run ads over time. Use Search Terms screen to find long tail versions (cheaper and more targeted) of your converting KWs that cost more. Use high intent Keywords, with buyer urgency. Not every KW can or will be a 10/10 QS. Sometimes, there is nothing you can do about a lower quality score (competition and or lower relevancy according to Google).