What does "Can't send message on card 1, error 21" mean? I'm trying to send an SMS with Messenger on my new Nexus 5X running Lollipop, but sending always fails with Can't send message on card 1, error 21.  Receiving SMS and making phone calls works fine.  What's going on?
Most likely the destination phone number is formatted wrong, which can happen particularly when importing contacts via third-party tool.

In this particular case, I used My Contacts Backup for iOS to export my contacts to VCF, then imported from Gmail.  In the process, some numbers were wrongly formatted as +61 0 4xx when they should be just +61 4xx, causing the highly uninformative error message above.  (The 21 error code actually comes from the operator's network, and has nothing to do with Android as such.)  Removing the extra 0 solved the problem.