Select PDF is not working, throwing an error The type or namespace name 'are you missing a using directive or an assembly reference?'

I am working on converting HTML string to PDF and after that Memory stream. I have used Iron PDF package but it is not free, not works for me. I have checked with free SelectPDF package but it is throwing an error 'The type or namespace name 'type/namespace' could not be found (are you missing a using directive or an assembly reference?)' after installing using nuget package manager.How to fix this issue? Or is there any other free nuget package to convert html to pdf?