How could I enable Sandbox for Care Identity Service (CIS2)?

I am developing a feature to login with NHS care Identity.
I tried to develop indirect request.
for example,<redirect_uri>

but I got the following error after go the link

invalid_client Client authentication failed

I believe it is because my sandbox app does not have access right to Care Identity Service (CIS2).

I already added Identity Service (Sandbox) in Connected APIs. However, it seems it does not enable Sandbox for Care Identity Service (CIS2).

ref: indirect request

Have you onboarded with the CIS2 Authentication service?

@wchoi - did you get this sorted?

noticed that you mentioned that you connected the Identity Service (Sandbox) , Do you need to use the Developer environment?
Or did you need to do something else ?