You purchased a Web-only plan that does not include access to Sejda Desktop.
https://www.sejda.com/upgrade

Here are plans with Desktop access:
https://www.sejda.com/desktop

Once you've purchased a Desktop plan your Web-only purchase will be automatically refunded.

The Week Pass plan is a one-time charge, not a recurring subscription. It does not automatically renew and downgrades to free after 7 days.