Re: " iMessage doesn’t allow me to contact some people why is that?
‘Can’t send message. iMessage needs to be enabled to send this message’. "
There are a few different possible causes, so it is hard to guess in your case,
(1) iMessages go by wi-fi between Apple to Apple devices, yet sending SMS between Apple and Andriod devices requires cellular-data to be turned on (NB: charges may or may not apply depending on your provider)
See: View or change cellular data settings on iPhone - Apple Support
(2) Wondering whether there are parental controls in place, or restrictions set in ScreenTime.
If needed: See how to: Use Screen Time on your iPhone and iPad - Apple Support
Else: (3) You could check whether you have blocked messages from those not listed in your Contacts app,
See: Screen, filter, report, and block text messages on iPhone - Apple Support
and
check whether or not you have a "Contacts" card for the individuals you can not send to or receive from.
See how to: Add and use contact information on iPhone - Apple Support