So the issuer you are currently trying to use is the Org Authorization Server, which is the one that issues access tokens you will not be able to validate. More details about this here: Signature Validation Failed on Access Token | Okta Help Center
In order to validate these tokens, you would need to use a Custom Authorization Server instead, which you may not be able to use in your org as it requires an additional API Access Management license. You’ll know you have the required feature if you can navigate to Security → API → Authorization Servers. This tab will not exist if you do not have this feature and it is where these custom servers can be created and modified.
You can read more about the different authorization server types here: Authorization Servers | Okta Developer