How can I apply for OPT EAD with a pending I-485?

It is up to the USCIS to decide, but they have refused issuance of a second EAD with a pending adjustment. It makes no sense to argue that you need training to go back home when you already filed an adjustment. Best not to apply.

It''s not clear to me why you would want to do that.

Often USCIS will not wish to give a person two EADs at the same time.

We generally advise clients to maintain non-immigrant status during the pendency of I-485, when doing so is possible. You have already obtained an EAD based on the I-485 so you should discuss with your attorney if there is any benefit to getting an F-1/OPT. The reason you would do it is to protect yourself in case there is an issue with the I-485 and I-140 (which you state is pending).

You are not allowed two EADs at the same time so you need to pick one.