If you only have created one database, actually you have two database, master db and other database. That's same with me:
![enter image description here](https://i.stack.imgur.com/I1JiE.png)
You can not choose the same database as the member database, only the master db you can choose.
Add member database:
![enter image description here](https://i.stack.imgur.com/aBenl.png)
SQL Data Sync is a service built on Azure SQL Database that lets you synchronize the data you select bi-directionally across multiple SQL databases and SQL Server instances.
This means that you can sync the data between the database in same Azure SQL server or other.
![enter image description here](https://i.stack.imgur.com/RCItC.png)
For your question, You want use the same database for metadata and my sync data,
I think you may think about using Azure Data Factory, Data Factory support you choose the same database both as source dataset and Sink dataset.
Here's the tutorial: Copy data to or from Azure SQL Database by using Azure Data Factory.
I think it can help you achieve your purpose.
Hope this helps.