Hello Azure Account Owner,
Thank you for posting your question in the Microsoft Q&A Community.
I understand that you are not able to create a New Identity Validation within your Trusted Signing Account.
Please be informed that you need to be assigned the Trusted Signing Identity Verifier role to be able to perform this operation. This is a role specifically created for the operation.
Follow this link to get more information https://zcusa.951200.xyz/en-us/azure/trusted-signing/concept-trusted-signing-resources-roles
Being assigned an Owner role with full permission on the Subscription won't grant you access as seen in the screenshot below.
After assigning the user account to the Trusted Signing Identity Verifier, I can create a New Identity
Let me know if this helps.
Babafemi