Problems connecting your WordPress site to Jetpack.
XML-RPC server accepts POST requests only
After setting up a fresh install of WordPress on my new Digital Ocean box with PHP 7.0 I had a few issues reconnecting my Jetpack accounts.
I got the error ‘XML-RPC server accepts POST requests only’ and after doing some research found out it’s caused by a missing PHP extension for XML that handles Dom activities.
On Ubuntu run the following:
sudo apt-get install php-xml
After installing and restarting my server the problem was resolved. One thing the Jetpack documentation didn’t mention as a possible cause for this error was the fact that the extension was or could be missing.