It depends on the role you’re applying for.
Employers choose whether they require a cover letter, make it optional, or don’t require one when they post a role on Oliv. So, if the role requires a cover letter, you wil need to write one in order to proceed with the application.
Having said that, you should know how to write a great cover letter regardless of whether you’re reuested one or not.
More on writing a great cover letter here.