Impact of non filing 9C

For F.Y 2018-19, if your turnover is more than 2 Crores than its mandatory to file Form GSTR 9 Annual return... otherwise it's Optional.
GSTR 9C is mandatory only if your Turnover is more than 5 Crore for F.Y 2018-19.

So, If your turnover more than 5 Crores, than both GSTR 9 and GSTR 9C is compulsory for you.