My iPad is lost/stolen. It doesn't appear on my Find My iPhone. So I cannot erase. What should I do?

If your iPad doesn’t appear in the Find My App, assuming that both your iPad and iPhone are signed-in to iCloud with the same AppleID, then this would indicate that Find My was not enabled prior to its loss.

Here is Apple’s advice for lost or stolen devices:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support

Use the Find My app to locate a lost or stolen device – Apple Support

If the Find My iPad feature was enabled prior to it being “misplaced”, then if active, you would stand a good chance of locating it. 

However, if the feature was not activated prior to being misplaced, or the iPad has been inactive for more than 24 hours, the last known location will not be available - and the opportunity to use technical means to recover the iPad have been lost.

Other than reporting the loss/theft to your local authorities, there is very little that you can do.