My iPad was stolen and I can’t turn on Find My iPad on my iPhone

You can only turn on Find My on the device that you want to track BEFORE it is lost or stolen.

If you actually could locate it how could that help? Confront the thief? People have been murdered doing that. Ask the police? Unless you live in a very small town police don’t want to get involved with low value thefts that have very little chance of recovery. Your iPad is probably already on its way to a repurposing factory in Shenzhen, China.

Here’s Apple’s advice→Find your lost iPhone or iPad - Apple Support. You will notice it doesn't address stolen device recovery, just how to protect your contents; your chances of getting it back are nil.