css style sheet not found

I have a website https://joinpakarmy.com.pk I have purchased shella theme but when now I’m trying to install this theme but an error occurs that the CSS style sheet not found … so guide me

can you give me a solution for this error my website is joinpakarmy.com.pk

and my other website is https://selection.pk/ pls see and suggest me the solution

Are you sure you bought the WordPress version? If yes, then please follow link shared by Charlie above.