I am new to such A/B partition system. However,
""PS J:\Lenovo Z5> adb reboot bootloader
PS J:\Lenovo Z5> fastboot flash recovery twrp.img
target reported max download size of 536870912 bytes
sending 'recovery' (54053 KB)...
OKAY [ 1.214s]
writing 'recovery'...
OKAY [ 0.005s]
finished. total...