That link was for the web client, which was discontinued months ago and no longer works.
The client we're currently working on full-time is the mobile app.
You can download it from the Zapstore https://zapstore.dev/apps/naddr1qvzqqqr7pvpzpklqkxldpq25w5qa4hlp7sx636su084zqg2p6skc446x7rav5w56qqfxuet5wahhy6ewd4hhxarjduhxzurssulxpx
or directly from GitHub