Hello Pavankumar-3526,
Welcome to the MS Q&A platform.
To enable Appendonly after you enable the synapse link, you need to unlink the tables from the manage tables section and re-add it again with the append-only option.
The other option is to unlink the Azure Synapse link and re-enable it with the append-only option on your tables.
I hope this helps. Please let me know if you have any further questions.