I Have an OnePlus 8 5G and it’s Not temporally unlocked And I request the permanent Unlock by the T-Mobile app it get me an error (SERVER ERROR: ERROR CODE=255)

This is a peer help forum.  No one here has access to your T-Mobile account or the ability to unlock your device.  Try reaching out to the T-Force Team via Facebook or Twitter to see if someone there can help you get it unlocked.  You must meet all of the unlock requirements so review this before reaching out to them.  SIM Unlock Policy | Unlock Your Mobile Wireless Device (t-mobile.com)