Password protection is available for Assets, Collections and Folders when you want to share content externally with others over a link. You can find more about that here
Frontify does not offer password protection for a Guideline, Library or a Project. Instead, your Guideline, Library or Project is only available to the users you invite to View or Edit the content. Once a user is invited to a Guideline, Library or a Project, they need to create their user account with email and password to access it. Then by default, they see all the content in the Guideline,Library or Project. Only the invited users can see the content. You can see how to invite the users here.
Another possibility would be to add password protection to the files before you upload them to Frontify.