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Hello,
The elevation of the city I am in is 930 meters, and because of this, the 21 psi starter map only delivers a maximum of 18 psi. I tried to fix this, but I haven’t been able to succeed in any way.

If you're at elevation the absolute pressure target should be lower. Read up on how relative pressure works.

So because I'm at altitude I can never get 21 psi of pressure?

If your elevation is why your absolute pressure target is lower and you read up on relative pressure you'd understand that you are indeed making 21PSI of positive pressure, relative to atmospheric pressure.

Is it possible to open 21 psi pressure by changing the setting?

If you want to ignore that you're already making 21PSI positive relative pressure and want 21PSI absolute pressure instead you'd actually be targeting more than the turbos efficiency range. You do have control of the turbo targets in the software.