@Dev, Surya Welcome to Microsoft Q&A forum and thanks for reaching out here.
The error message you are seeing indicates that the login for the service account is failing when trying to connect to the Azure Synapse dedicated SQL pool using the pyodbc library in Synapse spark.
Have you double checked if that service account has necessary permissions to establish a connection to your dedicated SQL pool? If not, I would recommend verifying that first.
Let us know how it goes.