F5 blocking my webpage that works as monitor of Sites hosted behing F5

Obviously you have an F5 ASM configured with Bot Defence - it stops bots (including your monitoring website/script ) from getting through to the backend server because the requests are sent by a script/bot and not a human.

ASM Bot protection works by injecting a piece of encrypted JavaScript into the responses and this JavaScript detects things like mouse moves and keyboard strokes. A real human behind a web browser will be moving the mouse and typing on the keyboard during the session, a robot just send an automated HTTP requests.

If you want your monitoring website not to get blocked then you should configure the Bot Defense whitelist, documented here:

https://techdocs.f5.com/en-us/bigip-14-1-0/big-ip-asm-implementations-14-1-0/configuring-bot-defense.html