rooting: recovery mode does not work

I have followed these steps to try to root my phone. Then I follow this guide, but I'm stuck where it says "Step 2":

I am trying to access the recovery mode on my PC with the adb reboot recovery command, but my smartphone shows the message and "No command" icon on the screen and after a while it just reboots normally.

I just wanted to install Magisk, so I did.

C:adb>adb reboot-bootloader

C:adb>fastboot flash recovery recovery.img
target reported max download size of 536870912 bytes
sending 'recovery' (21104 KB)...
OKAY ( 0.491s)
writing 'recovery'...
OKAY ( 0.235s)
finished. total time: 0.736s
C:adb>fastboot reboot

finished. total time: 0.010s

and then I expected to be able to enter recovery mode, but I tried every possible key combination and I can

Actually, before the "No command" screen I get a warning message asking me to visit:

Also, my goal is to install Magisk, so if there is another way to do it you will be welcome too.