I want to know is there any drawback of storing code on Github private repo
Storing your code on GitHub is no less secure than storing it anywhere else. For example, GitHub generally takes significant effort to secure repositories, and staff are not permitted to look at the contents of private repositories without the consent of the repository owner. Pushing this code to GitHub will not intrinsically expose it any more than storing it on any other server.
However, having said that, storing credentials in your repository is a security problem regardless of where you host that code. It is easy for a repository to accidentally leak for many reasons, due to server misconfiguration, laptop theft, or various other situations. You would be well served to put at least a modicum of effort into using a more secure practice for storing credentials, if for no other reason than that you will have them stored in a single, secure place where you can find them all. For example, rotating credentials is much easier when they all live in a tool like Vault and you can easily rotate a compromised credential across all systems.
So, in general, what you are doing is not very secure, but using or not using GitHub will not change that.