You need to do one of:
- Install the certificate for SQL Server as in your Trusted Certificates store.
- Select the connection option "Trust Server Certificate".
The latter is less secure, but if this in your home environment, there is not really an issue with it. (If you would be working in a bank or a hospital with some really sensitive data, it would not.)
Exactly how you set this option in Power BI desktop, I don't know. For help with PowerBI a good place is https://community.powerbi.com/t5/Forums/ct-p/PBI_Comm_Forums”