First: try to press power buton when this error appears. I can happend that it will continue to boot!
This is what happened to me when I installed the original PIE ROM!
Second: if above is not working, nor any button combination: VOL UP+Power or VOL DOWN+Power then we need the emergency tool!
Download it from
https://www.getdroidtips.com/nubia-emergency-tool-unbrick/
You need also the TWRP recovery in 7z format:
http://www.mediafire.com/file/go4cj75ym8e5m57/twrp-3.2.3-x_blu_spark_v9.85_redmagic.img.7z/file...copy it to c:\Program Files (x86)\nubia\BackupRecoveryTool\Recovery\
Now start the emergency tool with Administrator rights and proceed with phone connection procedure: Press vol+ and vol- and power same time, keep pressing at least 15 sec then connect the cable with pc and phone. release buttons and wait for awhile. With luck it will connect with emergency tool and you can try to flash a recovery (TWRP in our case) an than you can flash a custom ROM (or an original one).
Press the recovery option and chose the recovery that you copied as I wrote above. (normally the other recovery for other nubia phones are in password protected 7z format but Red Magic is not supported yet!)
it will try to open but will give an error. Now copy the recovery.img from here:
https://www.mediafire.com/file/fpa7enbjzhhqxxt/recovery.img/file to c:\Program Files (x86)\nubia\BackupRecoveryTool\ and pres the try again button till it will start to communicate with phone.
Let it work for at least 5-8 min.. Maybe there is no progress but it will work (at least for me!).
Then you can disconnect the phone and try to go in recovery with VOL+ and POWER. From here I think you now what to do! (remember: in TWRP mode the phone is in MTP mode and you can connect the phone with PC an copy your ROM from PC to Phone!!! and then you can install it through TWRP).
Good Luck :fingers-crossed: