Cannot send messages to one specific person

I am using a IPhone SE with the latest update. Usually I text this specific person frequently but now I can’t send messages to them. With other contacts i’m able to send messages without any problems but this person i’m able to recieve messages but I’m not able to send them. I already tried the settings and changing it to send as SMS but it didn’t make a difference since the other person already had an android and I’ve alre tried resetting my phone but that didn’t work either. How can I fix this?