Mac OS for mid 2014 MacBook pro

Big Sur. See the following from Identify your MacBook Pro model - Apple Support

MacBook Pro (Retina, 15-inch, Mid 2014)

Model Identifier: MacBookPro11,2, MacBookPro11,3

Part Number: MGXC2xx/A, MGXA2xx/A

Newest compatible operating system: macOS Big Sur 

MacBook Pro (Retina, 13-inch, Mid 2014)

Model Identifier: MacBookPro11,1

Part Numbers: MGX72xx/A, MGX82xx/A, MGX92xx/A

Newest compatible operating system: macOS Big Sur

If not already upgraded to that, see: How to download macOS - Apple Support

If you have Big Sur but not the latest (11.7.2) you can update to that Update macOS on Mac - Apple Support