You CSS expression is indeed incorrect,
form is a tag (it can also be an attribute
role in your case), not an id. Hence, you should write
Regarding your previous issue with XPath, it can be something to do with charsets. Did you try something like
//input[contains(@placeholder, 'Enter admin name'])?
Since you are using Chrome, you can manually test your locator using Chrome Developer Tools:
document.querySelector() for evaluating a CSS locator
$x() for evaluating a XPath locator