Importing Self-signed certificate into Keychain Access for development work

I found the solution in an old post, which I have not idea how to properly reference (thank Apple for not making it easy to cross-reference help posts /sarcasm) titled "Importing P12 into Keychain access fails due to incorrect password".

This got me past the Password challenge. But now the cert import fails with "Error: -26276" and we've been seeing a lot of this, but this cert should import. I can't find a reference to what this error even is. Has anyone else seen this? Has anyone else figured it out?

This .p12 cert and private key pair imported into Firefox on Mac without issue. It seems like this should work. Thank you.