Thank you for using Apple Support Communities! I understand that the Send Read Receipts option is missing on your iPhone 7 Plus under Settings > Messages. You've come to the right place for help.
I would like to start by having you open a conversation thread and tap the name of a contact at the top of the conversation. Then tap
If the option is missing from that screen as well, force quit the Messages app and restart your iPhone:
Keep us posted on the results.
Cheers!