Step 1. From Flathub download either ProtonUp-Qt or ProtonPlus both of them will do

Step 2. Open either ProtonUp-Qt or ProtonPlus

Step 3. Download and Install Proton-Ge

Step 4. Restart Steam if open

Step 5. Right Click on Lego Batman 2 in you're steam Library then select Compatibility

Step 6. Force the use of a specific Steam Play compatibility tool then select Proton-GE

Step. 7 Play the game and Enjoy one of the greatest lego games ever made