Are you trying to configure this in ?
Many VTXs come "locked" to legal frequencies/power. You must apply an unlocked table or use the manufacturer’s instructions to unlock it, otherwise, you cannot use high-power modes. Reboot: Always save and reboot after applying a VTX table. To get you the exact answer, I need to know: What is the exact model of your VTX ? What protocol are you using (SmartAudio or IRC Tramp)? vtx-tables
Without a correct VTX table, you cannot change your video channel or power output via your radio (OSD/stick commands). If the VTX table is wrong, it may revert to default settings (often 25mW) every time you plug in your battery. How to Configure VTX Tables (Betaflight) Are you trying to configure this in