I tried creating custom post using custom post UI and advanced custom fields plugins on localhost but When I set everything up and click on view to see my project page(portfolio item) on front end, it says I have to connect to internet, and when I do that, it just says : error. How can I fix this?
If you create a Custom post type you have to create also 2 files. The single post page and the archive post page. Without those 2 files, WordPress does not know how to display the content of the page you are trying to view. It is not for beginners and I would recommend Elementro Pro to create the front end if you are a beginner. If you want to create manually would need some PHP, HTML, and CSS knowledge.