SWAN keeps track of notebooks that you trust. Every time you start a new SWAN environment you must re-trust the notebook.
To trust a notebook you must first open it and you will see a button on the top right indicating that the notebook is "Not Trusted". Click "Not Trusted" and confirm that you like to trust the notebook.
Once a notebook is trusted you will see on the top right that the notebook file is "Trusted". And can now execute code within the notebook.
This feature exists to ensure you do not run potentially malicious code that may have been sent or shared to you.