Multi site tracking & issue with Facebook pixel

Greetings Circle Community! I hope you are all keeping well. I'm a UK member and this is my first post. I'd really appreciate your help solving an issue I'm having. Here's a summary...

I have a client with two websites, one on Squarespace and another on a different platform. I can connect both URLs to the same Facebook pixel HOWEVER when I go to set up Custom Events for the Squarespace URL it won't let me...the pop-up box comes up briefly and then disappears leaving the message 'Action not available'.

How do I overcome this issue?

Thank you to anyone who is able to assist.