The link from the suggestion email no longer works
Short answer
The one-click links from the suggestion email are valid for one week. After that This link has expired appears. The suggestions themselves are not gone — you handle them in the app under Sources.
Details
The links work without signing in. For that to be possible they are signed and time-limited: a link that lasts forever is an entrance to your Workspace sitting in every forwarded mailbox.
On the expired page, Open the Sources page takes you straight to where the same suggestions sit with the same three decisions.
Two other causes
The suggestion is already handled. If someone else added, hid or rejected it, Picasi says so instead of accepting a second decision.
The bulk accept finds nothing. With Add all green it then says No green suggestions remain — they have all been handled.
If that does not help
If no email arrives at all, check the recipient list under Workspace settings → AI source suggestions.