Various One Plus 3 and One Plus 3T users faced and still facing dm-verity warning when they try to flash/sideload stock rom or Custom Rom with TWRP recovery. This fix is only for those users who are rooted and installed TWRP recovery .If you want to root and use TWRP to flash new roms then See this post .
This method is reportedly working on both One Plus 3 and 3T . This fix is provided by xda senior member
How to disable dm-verity warning on One Plus 3 and 3TIts very simple . If you are rooted then you probably had used fastboot to unlock and flash twrp . This time we have to use fastboot mode again because we have to write two fastboot commands to fix this warning/error.
Boot into fastboot mode and connect your phone with USB cable to computer .
Enter These Commands One By One -
fastboot oem disable_dm_verity
fastboot oem enable_dm_verity
Note that you have to type both commands if you type only first command then the fix may or may not work .So its recommended to enter both commands .
You can see this xda thread where this fix is provided.