The Profile URL import requires Goodreads' RSS feed, which can be blocked for app requests. Here's why and what to do.
Why it happens
Goodreads shut down their public API in 2020. The Profile URL method uses their RSS feed, but Goodreads sometimes blocks RSS requests coming from servers (as opposed to a real browser). When this happens, the import fails with an error even if your profile URL and privacy settings are correct.
The reliable fix: use CSV instead
The CSV import is the most reliable way to sync. It doesn't depend on Goodreads' servers at all. See the "Import books from Goodreads" help article for step-by-step instructions.
Make sure your profile is public
Even for URL sync attempts, your profile must be public. In Goodreads: Account Settings → Privacy → "Who can see your profile" → set to Anyone.
The URL sync may work intermittently. If it fails today, try CSV — it's faster and more complete anyway since it includes all your shelf data.