How to install macOS 14 on a 2018 MacBook Pro?

Are you sure about the model year? You can double-check it in  > About This Mac.

A MacBook Pro 2018 is compatible with macOS Sonoma. If you can't upgrade, try these various solutions:

1. Download the upgrade from the App Store : App Store - macOS Sonoma.
2. Start your Mac in Safe Mode and do the upgrade: Start up your Mac in safe mode - Apple Support.
3. Start your Mac in recovery mode using the Option-Command-R key combination. This should offer you the latest version of macOS compatible with your Mac. For more informations: How to reinstall macOS - Apple Support.
4. Create a bootable key to install the upgrade: Create a bootable installer for macOS - Apple Support.

If that MBP has a hardware model year of 2018, you can upgrade it to either macOS 14.* (Sonoma) or macOS 15.* (Sequoia), but not macOS 26.* (Tahoe).

If it is a 2017 model that you bought in early 2018, before the 2018 models came out in July, you can't upgrade it past macOS 13.* (Ventura).