The expression you have used is not working for the condition. In the end it must evaluate to true or false.
In the example Cloud mask - SNAP sentinel 2 - #52 by marpet I’m simply generating a new band by using the Band Maths. IN this case Mosaic is not necessary.
For the Mosaic condition you can use an expression like
not (scl_cloud_high_proba || scl_cloud_medium_proba || scl_thin_cirrus)