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Problem:

I have logged in to the SQL Server using winodws authentication and created a new login with SQL authentication. Then tried login with the new login and got the error as login failed for the user even though the username and password that I entered was correct. This is the error message I got.
Cannot connect to server ip address. Login failed for user ‘sql-user’. (Microsoft SQL Server, Error: 18456)
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Solution:

The problem was not with the incorrect username or password. The SQL Server was configured with only windows authentication mode.
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You can change the Server authentication mode via UI or using the below script in T-SQL. This script updates the registry. You need to have sysadmin role to make this change. 
USE [master]
GO

EXEC xp_instance_regwrite N'HKEY_LOCAL_MACHINE', N'Software\Microsoft\MSSQLServer\MSSQLServer', N'LoginMode', REG_DWORD, 2
GO
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Once the changes are done, the SQL server needs to be restarted to be able to login via SQL authentication mode.
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