I do not remember anything like 'Creating /data/adb/magisk_merge.img'. That was long time ago.
I have installed Magisk-v15.2(1520).zip file (size 5.8Mb) from TWRP. Now, how I see, there is a Magisk-v16.7.zip (size 4,0Mb)
Maybe you should try with that old version.
Edited: I just checked on my...