Google is informing me that my audience size is too small to run my retargeting campaign?

When setting up your remarketing campaigns on Facebook and Google, you may be informed that they are ineligible due to being "Too Small to Target". This is normal and common when you first set up your remarketing campaigns. 

It's just telling you that the retargeting audience is too small for Google to use at this stage (but this will grow over time as you send more traffic to your store).