How can I solve the issue of “at rule or selector expected”?

 In the context of Salesforce, you can resolve and troubleshoot the issue of “at-rule or selector expected” by using the following steps below:-

Review the CSS code:-

You can go through the CSS file where the error occurred and there you can look for any error related to syntax or misplaced characters.

Checking for missing or extra characters

Try to make sure that all opening and closing braces are properly matched and there should be no missing semicolon at the end of every declaration.

Verify the context of the error

Try to determine the context of the error in which this particular error is occurring.

Use a CSS validator

Try to use a CSS validator tool or even browser developer tools for validating the CSS syntax and identifying any errors automatically. This would help you in catching syntax errors that might be easy to overlook manually.