Hi Spoorthi,
Mendix publishes your application without your local data to the sandbox, your database in the sandbox is completely empty. So the data you have locally is not the sandbox. As there is no data, you also do not have an MxAdmin account, as that in the end is data in your database.
If you want to be able to login and do all necessary computations I would recommend you add your logic (for example your admin creation) in an afterstartup microflow of your app.
Hi Spoorthi ,
To login in sandbox with admin credentials you have to use generated password, which you can find under the project security settings , you can see below , there is option to copy password to clipboard
regarding the data not visible issue, you have to generate your data in sandbox as it will be empty dataset, local data won't be visible there.
Good luck!