Tablet [Q] Question about RO /system/bin

Hello XDA need your support.I am trying to fix a bootloop but I cannot run the ADB SHELL - exec '/system/bin/sh' failed: No such file or directory (2) - when I try to insert the sh file with adb push it sends me Cannot Copy "Read-Only" system

I know I can unlock the /system with the shell codes but my urgent question is how I can insert this sh file because android cant find it

And the most important is then the ADB SHELL is needed for fastboot commands

Any Ideas ?