The reason you are seeing this message is because you have selected the 'Automate my authentication and skip associated pop-ups' option in Quick Settings.
Lean Library browser extension is applying QUB Library proxy to all URLs you are trying to access.
To remove this setting, you can un-select this option.
The reason you are seeing this message is because Lean Library cannot detect a DOI on the webpage.
If there is no DOI or ISBN on the webpage Lean Library extension will not be able to extract the data and search the Library's holdings.