Azure AD B2C: Clients must send a client_secret when redeeming a confidential grant

I had the exact same issue as you and was just able to resolve it.

AD is requesting the client_secret from you, because it isn't configured for PKCE yet. To tell AD that you want to use PKCE for a specific redirect url you need to set its type from 'Web' to 'Spa'. This can be done in the manifest.

Search for the property replyUrlsWithType in the Manifest and look for your .../signin-callback url. Change its type to 'Spa' and you should be good.

eg.:

"replyUrlsWithType": [    {        "url": "http://localhost:8080/signin-callback",        "type": "Spa"    },]

The configured url will now disappear from your Authorization page but thats ok -> it's still present in the Manifest. The MS team is working on this new type.

Also make sure you marked your application as a public client.

For more information, see my answer here: Is Active Directory not supporting Authorization Code Flow with PKCE?

In the Azure AD B2C App there is now a simpler option to do that. In the Authentication tab where the Web Redirect URIs are you will probably see a message This app has implicit grant settings enabled. If you are using any of these URIs in a SPA with MSAL.js 2.0, you should migrate URIs. When you click that, a new window will let you choose which Redirect URI you want to move to the SPA Redirect URIs instead:

After that just click Configure and it should work. The Redirect URI will now be located in the SPA section instead of the Web one.

Your image shows a CORS error.

I'm not sure if oidc-client works OOTB with B2C. It's more for identityserver.

Have a look at the msal.js sample.