How to set the default argument to an array

The reason this happens is that you have specified that your Parameter $games is a string, what you want is an array of strings: string[] so your code becomes:

param (
    [string[]]$games = @('Subnautica', 'Gearcity')
)

This bit: [string]$games forces your variable to be a string. To make it an array of strings you need to add[] to variable type declaration.

[string[]]$games = @('Subnautica', 'Gearcity')