Most often the link was copied incompletely or opened in a browser instead of the app. The subscription link looks like https://subscription.obfsn.com/c/<token> and belongs inside a VPN client, not in a browser's address bar.
Tested apps and detailed steps for each platform are on the setup page.
In a browser you see configuration text or a page with the file contents. No harm done: copy the address from the address bar and paste it into the client as a subscription.
Forwarding through messengers and notes puts spaces and line breaks into the address. Send the link to yourself as one message and paste it whole, from https:// to the last character.
The subscription link has a device counter. Open the Devices section, look through the list, unlink what you no longer use and import again.
Check the payment page. The 7-day trial is issued once; if the period has ended, the subscription needs extending, otherwise the client gets an empty answer.
On Android TV and Apple TV a link cannot be opened in one tap, so the subscription is added through the clipboard: copy the link on a phone or computer, forward it to yourself on the TV and add the profile from the clipboard in the app.