Hello joaocnpbarroso,
If your USB-C to 3.5 mm Headphone Jack Adapter is not working on your iPad Pro, then we would first recommend following these steps:
1. Check for debris in the headphone port on your iPhone, iPad or iPod touch.
2. Check your headphone cable, connector, remote, and earbuds for damage, like wear or breakage.
3. Look for debris on the meshes in each earbud. To remove debris, gently brush all openings with a small, soft-bristled brush that's clean and dry.
4. Firmly plug your headphones back in. If your iOS device has a case, remove the case to get a firm connection.
These steps and more can be found here: If your headphones don't work with your iPhone, iPad, or iPod touch
Also, let's update your iPad to the latest version of iPadOS, 15.4. This shows how:
1. Back up your device using iCloud or your computer.
2. Plug your device into power and connect to the internet with Wi-Fi.
3. Go to Settings > General, then tap Software Update.
4. If you see two software update options available, choose the one that you want to install.
This information is from this article: Update your iPhone, iPad, or iPod touch
If this continues after looking over those articles, then we would advise having the adapter and your iPad inspected for service.
iPad Service and Repair
Take care!