@HighScoreJimmy Not too familiar with the Mortal Kombat series, but a quick search says she was released Nov 26th. which is pretty recent. I'm assuming Stadia games has different release timeline compared to the other platforms. Hopefully it will be available on Stadia soon.
I bought the Premium Edition and she is not there, so like Ivan said we're probably behind updates compared to other platforms.
@HighScoreJimmy
Sindel is now added to the game. Next time you boot it up she should be on the character select screen and playable.