Hi Vijaya,
There are two options why you get this error.
1. there is an error in the SCSS file
2. The file that Mendix studio is looking for is not at the correct location.
In case of options 2, copy the scss file from the location where it was in version 8.12.1 and copy it to "...\your_project\theme\web". Deploy your app again and check the results.
Hope this will help,
Jan
Go to App > Select Synchronize App Directory
Then run application again