Most likely, the email has ended up in the spam (or junk) folder. If your referee can not find your invitation email there, you might have misspelled the email address. Check on your application dashboard if the email address you have entered for your referee is correct.
If the email address is wrong, simply add a new invitation with the correct address.
If the email address is correct, you can send a reminder email from your application dashboard. Note that the number of reminders you can send is limited.
In general, we highly recommend that you inform your referees before inviting them.
In case this issue persists, please contact the admission office of the graduate school you are applying to.