I am a senior Siebel CRM developer having more than 8 years of working experience. Now, I am very keen and excited to learn Salesforce and get certified as soon as possible. 

Please guide me where to start from scratch ?
['https://salesforce.stackexchange.com/questions/213173', 'https://salesforce.stackexchange.com', 'https://salesforce.stackexchange.com/users/55249/']
After some research and not reaching to any solution, I refreshed the selected Business Units by removing them, saving and the adding and saving again. This solved my issue.
You may need to raise the case with SFMC support. I have seen these issues, and support needs to toggle some backend settings to reset the SC and SFMC connection.