Can't flash custom TWRP recovery image on Motorola Moto G7 Power phone
G7 Power, being a new device, has the new A/B partition layout, requiring new installation procedures that differ from standard A-only devices. This is clearly outlined in the thread where you got your image from:


  This device has 2 "slots" for ROMs / firmware. TWRP will detect whichever slot is currently active and use that slot for backup AND restore. There are buttons on the reboot page and under backup -> options to change slots. Changing the active slot will cause TWRP to switch which slot that TWRP is backing up or restoring. You can make a backup of slot A, switch to B, then restore the backup which will restore the backup of A to slot B. Changing the slot in TWRP also tells the bootloader to boot that slot.
  
  Decryption only works when TWRP is permanently installed.
  
  Installation
  
  To temporarily boot this recovery: 
  
  fastboot boot twrp-version-build-river.img
  
  To permanently install it:
  
  
  Temporarily boot TWRP
  Put the TWRP image in your external SD Card or, in case you don't have one, push it to /data with adb this way: adb push twrp-version-build-river.img /data/
  Tap Advanced -> Install Recovery Ramdisk -> Navigate to /data or /external_sd and select TWRP -> Swipe to Install
  If you previously installed Magisk: Select Fix Recovery Bootloop from Advanced to fix/avoid recovery bootloops
  Done! Optionally you can delete TWRP from /data or from your external SD Card now.
  


Read more carefully next time...