If your issue still exists , you just have to flash the original recovery via fastboot , then volla os should start as usual.
its a known problem after updating to the newest android 9 version on volla .

u can get the original recoveryimage here:
https://volla.online/en/faqs/

under "how can i update the operating system volla os " you can find a link to the recovery image and further instructions
how to solve the problem.