"Remove ID" option not working when I try removed Keychain Digital ID's

I set up a digital signature to sign a PDF form the other day.

Some people (clients) are now getting a "digital certificate invalid" message when they try and send me an email, so I really need this to be removed.

I am using Acrobat Pro DC

I am in the Identities and Trusted Certificates section.

I select the Keychain Digital ID that I am wanting to remove, but when I click "Removed ID" nothing is happening.

I seem to be following all of the steps I've seen explained in other forums, so I'm a bit stuck.

I am attaching a screenshot of the notification my client is getting when they email me.

How can I get this removed ASAP?