Login Error while connecting SQL Database through Power BI?
I found resolution for this error:

By default, the Encrypt connection check box is selected to signify that Power BI Desktop connects to database using an encrypted connection. If a connection to your SQL Server is not established using an encrypted connection, that Power BI Desktop prompts to connect using an unencrypted connection. 

Once you click OK, it will be directly prompted to turn off encryption. Or you can go to 


  File->Options and Settings -> Data Source Settings, choose your SQL
  Server data source and select Edit and uncheck "Encrypt Connections"
  option.


In addition, when we connect to SQL Server in Power BI Desktop, need to select the authentication mode to connect to the SQL Server database.
 

1) Windows: This is the default selection. Select this if you want to connect using Windows authentication. 
 

2) Database: Select this if you want to connect using SQL Server authentication. After you select this, specify a user name and password to connect to your SQL Server instance.

Note: I know it's foolish and so simple but works for me anyway @sepupic Thanks for useful comment.