Hey Simon! Thanks for the reply. This is fixed. The issue was related to your API key. For context, the write to table integration requires your Clay API key to function properly. The API key that was previously set as default was not the correct. I added your API Key (saved as "Clay Key") in a new column. You should be able to run the integration now - just check the fields mapped to confirm that's what needs to be sent to the other table.
Your Clay API key can be found in the settings section of your workspace. Click on your profile at the top right corner, select settings > select API key.
Let me know if this helps!