According to MS docs, Azure Stream Analytics currently supports three compatibility levels:

1.2 - Newest behavior with most recent improvements
1.1 - Previous behavior
1.0 - Original compatibility level, introduced during general availability of Azure Stream Analytics several years ago.

However, Bicep is aware of 1.0 | null only.

I got this warning: "The property "compatibilityLevel" expected a value of type "'1.0' | null" but the provided value is of type "'1.0' | '1.1' | '1.2'". If this is an inaccuracy in the documentation, please report it to the Bicep Team."

https://docs.microsoft.com/en-us/azure/stream-analytics/stream-analytics-compatibility-level