Z88TRXY I think it is more likely that this thread will receive responses if more information is provided. For example, what are you trying to accomplish (adding a certificate for what, that you hope will be used by what?), what steps have been tried so far, what did you expect to happen, what happened instead, were any error messages received?
Since this thread is tagged as "Pixel 6a" and "Pixel 6", is the issue observed on at least one device of each type?
What is the GrapheneOS build number that is in use?