I can't Install Ventura in mi iMac 2015 27 inches, what do I do?

You can't install Ventura or higher on an iMac (Retina 5K, 27-inch, Late 2015). Monterey is as high as that Mac can go.

The description of Final Cut Pro on the App Store Preview page says:

"System Requirements: macOS 13.4 or later, 8GB of RAM (16GB recommended for third-party effects and plug-ins), Metal-capable graphics card (Intel-based Mac computers only), 5.8GB of available disk space.

Some features require a Mac with Apple silicon. Some features require internet access; fees may apply. Blu-ray recorder required for burning Blu-ray discs."

So you can't run the current version of that application on your iMac. (There were old versions of Final Cut Pro that ran on Monterey, but I doubt if any of them are available to new buyers.)