Wiki page library cannot be created directly from the Site Contents - > Add an App option. However there is a workaround to create one.
In the below link/url, replace the tenant and siteurl properties and enter in a browser tab, you will be shown the option to create a wiki pages library. This is very useful when you want to migrate the on-prem classic pages to SharePoint Online
https://tenant.sharepoint.com/sites/siteurl/_layouts/15/new.aspx?FeatureId={00bfea71-c796-4402-9f2f-0eb9a6e71b18}&ListTemplate=119&
