help in connecting the sql server 2017 developer edition
It sounds like one of the following things is happening:


The sa account is disabled


either explicitly, or because you're in Windows Authentication mode

You don't know the password to the sa account


Since this is installed locally on your PC, you should actually be able to connect with sqlcmd without any parameters to use "Trusted Connection" to authenticate you:

sqlcmd


Once you're in, you should be able to check the authentication mode using an approach like this one:

SELECT CASE SERVERPROPERTY('IsIntegratedSecurityOnly')   
WHEN 1 THEN 'Windows Authentication'   
WHEN 0 THEN 'Windows and SQL Server Authentication'   
END as [Authentication Mode]


If that returns "Windows Authentication," then you'll need to go through the process of switching to mixed mode outlined here: Change Server Authentication Mode

If the query above returned "Windows and SQL Server Authentication" then it's possible that you have just mistyped or forgotten the sa account password, or it's disabled for some other reason.  In that case, you can run this code (also included in the docs link above):

ALTER LOGIN sa ENABLE;  
GO  
ALTER LOGIN sa WITH PASSWORD = '&lt;enterStrongPasswordHere&gt;' ;  
GO