Publishing fails - "Could not sign the air file. Unable to build a valid certificate chain for the signer."

Hi

I'm working on an AIR desktop project in Animate CC 19.1 build 349

The swf runs fine when I test it, however when publishing using a p12 cert, I get the error:

"Error creating files.

Could not sign the AIR file.

Unable to build a valid certificate chain for the signer."

I bought the Thawte code signing certificate, and followed the process to install it on my Windows 10 machine - purchase the cert in Firefox, picked up the cert in Firefox, and backed up the file as a.p12 from Firefox. Thawte tells me the cert is fine, otherwise it wouldn't back up from Firefox.

What are some ways to solve this problem?

Thanks for your time and help.