Below are proven solutions, ordered from least to most invasive.

Some users report that running chmod 755 mtk-su again from the /data/local/tmp directory and re-executing the binary multiple times can occasionally bypass a temporary glitch.

The error means that the MTK-SU tool attempted to force your MediaTek device into a vulnerable state while the device was already powered on or had a live USB connection. The third critical step—likely a memory negotiation or security token exchange—failed because the device was not in the expected "cold" (powered off) mode.