The message may be unavailable is a message. I often saw when engaging with the Meta App Dashboard with an be accessed at developers.facebook.com/apps However, this never prevented me from sending messages to an added recipient list which has a maximum of 5 using the test number.
To resolve this issue of messaging may be unavaiable. THere are couple of things that needs to be resolved such as:
If it is not resolved you can go futher and ask from assist ance from Meta Developer community (https://developers.facebook.com/community/) or Facebook developer support wihcih never responds to technical issues (https://developers.facebook.com/support/) or report a Bug on (https://developers.facebook.com/support/bugs/)