Versions:

Minecraft 1.16.3
Forge 34.1.0
Lost Cities 1.16-4.0.1-alpha

Minecraft crashes on clicking the "Create New World" button with the following information:

The game crashed whilst mouseclicked event handler
Error: java.lang.NoClassDefFoundError: net/minecraftforge/common/BiomeDictionary$Type
Exit Code: 255

Full crash log here.

Update forge. You're using a version that is too old

34.1.0 is the current recommended version for 1.16.3. What is the minimum version supported by this mod?

Go with latest. Recommended is usually not 'recommended'

you wouldn't mind listing what version and up you want people using, would you? or baking it into the JAR, which would be preferable? #329 (comment)