@Nes_Hdr Do you have Serverless enabled for your workspace?

1. Fine-grained access control on single user compute: Databricks Runtime 15.4 LTS does support fine-grained access control on single user compute, but it relies on serverless compute to run data filtering. This means that when a query accesses objects like tables with row filters or column masks, the single user compute resource passes the query to the serverless compute to execute the data filtering.
2. Error Message: The error message you are encountering, ROW_COLUMN_ACCESS_POLICIES_NOT_SUPPORTED_ON_ASSIGNED_CLUSTERS, indicates that the query on the table with row filters or column masks is not supported on assigned clusters. This suggests that the single user cluster you are using might not be configured to leverage serverless compute for these operations.
   
   In the documentation it was mentioned that Serverless needs to be enabled in that workspace. 
   https://learn.microsoft.com/en-us/azure/databricks/compute/access-mode-limitations#assigned-limitati...
   
   Hope this helps!