When attempting to write a text message on my Android, I received a message that said “Invalid recipient -- Unable to add this recipient. The recipient’s number is not valid.” when adding a contact to the message.
Though the phone number looked correct, the problem is often caused by spaces or invalid characters in the phone number.
Remove any hyphens, spaces or brackets from the phone number and just use all numbers to fix this problem.
You may want to open the Contacts app and correct any other contacts that have invalid characters. This is a consistent problem if you imported your contacts from another system or phone.