What happened
When a server-side check reaches a Steam login page, it has not obtained evidence about the recipient token. Calling that response an invalid link confuses the checker’s authentication with the address being checked. Your personal browser can be signed in while the server is not. Those sessions have different owners and purposes, so success in one browser does not authorize the other request.
What to check
Read whether the website says unavailable or explicitly rejects the recipient. Check the saved address for obvious format and ownership errors, but leave a correctly owned complete link unchanged. Report the authentication symptom to the service operator. A fresh check should be requested after the checker has its own functioning session, rather than after sharing your customer cookies or repeatedly generating new tokens.
Worked example
Suppose the link opens a trade page in your browser but the service reports that its checker is not connected. Your browser carries your own account session. The checker needs an independently authorized session to inspect the recipient page. Copying your browser’s authentication data would introduce account access risks and would not be an appropriate customer troubleshooting procedure.
Result and next step
The resolved result is a fresh recipient-specific response instead of a sign-in screen. Until then, the link’s live state is unknown, even if its syntax is accepted. Send the unavailable message and timestamp; keep login credentials out of the conversation. Review Steam account security advice and CASECS2 operation stages. Preparing a saved link does not prove that an item is deliverable or that withdrawal is enabled.