According to this moderator post
To update your trade URL go to the Gameflip main menu > Settings > Steam connect. Please note that you can only use the URL to the Steam account linked to Gameflip. We do not have the ability to unlink Steam accounts at this time.
Are you logged in Gameflip in the same account that you used Steam Connect?
Can you try changing it by using the Gameflip on Android/iPhone? You should see the Steam Connect or “Trade URL” page when trying to sell a CSGO item on the app.