Are you trying this with an iMessage that hasn't been delivered yet?Seems force to send SMS to someone who's on iMessage is no longer possible... Any ideas?
What about disabling iMessages in Settings before sending? Also you can try turning off cell data and Wi-Fi, it should send as SMS.
It's very annoying iMessage is linked to the sim card, and doesn't use a more flexible system like whatsapp, which allows you to use the same number even if you switch sim cards temporarily.
Can we not send an iMessage as an iCloud user instead of a phone number user?