You don’t mention what your user privileges are – are you an admin, editor, author, etc? An admin-level should be able to install plugins via Plugins, Add New.
Most new-to-WP people don’t realize that access to the folder structure (via cPanel or FTP) is not needed to admin the site. Any installation of themes/plugins is done via admin access, at the admin-rolelevel. Any adding of images to the uploads folder is done via the admin panel Media entry, or when adding/editing a post/page (assuming you have privs/ro;e to do that).
If your user is an admin-level role, then you should be able to add plugins (via Plugins, Add New), or add media (via Media page, or when adding/editing a page/post). If you aren’t an admin-level role (which is different than logging into the admin panel), then your capabilities are limited.
To understand roles and privileges, see https://wordpress.org/support/article/roles-and-capabilities/ .