User 'sa 'Login Failed. Cause: the solution is not connected to the trusted SQL Server.

This error occurs because SQL server uses the "Windows only" authentication method,
Therefore, you cannot use the SQL Server Logon account (such as SA) to connect to the instance.

The solution is as follows:
Method 1:

In Enterprise Manager
Expand "SQL Server group", right-click the name of the SQL Server
Select "properties"
Select the "Security" tab.
Under "authentication", select "SQL Server and Windows ".
OK, and restart the SQL Server service.

Method 2:

Modify the Registry to solve this problem:
1. Click "start"-"run", enter regedit, and press enter to enter the Registry Editor.
2. Expand the registry key in sequence and browse to the following registry key:
[HKEY_LOCAL_MACHINE/software/Microsoft/MSSQLServer]
3. Find the name "loginmode" on the right of the screen and double-click to edit the double-byte value.
4. Change the original value from 1 to 2 and click "OK"
5. Disable Registry Editor
6. Restart the SQL Server service.

Method 3:

If you still cannot connect to SQL Server in Windows Authentication mode.
This is because there are two default logon accounts in SQL Server:
Builtin/administrators
<Machine Name>/Administrator deleted.
To restore these two accounts, you can use the following methods:
1. Open the Enterprise Manager, expand the server group, and then expand the server
2. Expand security, right-click logon, and click New logon"
3. In the Name box, enter builtin/administrators
4. On the "server role" tab, select "System Administrators"
5. Click "OK" to exit
6. Use the same method to add <machine Name>/administrator to log on.