Booting into recovery mode on a Mac usually requires rebooting the computer and holding option + R. There are times where option+R is not working on certain keyboards, usually a Windows keyboard.
To boot into recovery mode on a Mac having this issue:
1. Open the terminal
2. Copy & paste in terminal: sudo nvram internet-recovery-mode=RecoveryModeDisk && sudo reboot
3. Press “Enter”
4. Your Mac will reboot into recovery mode