we get "The requested Payment Method is not available" error when we try to create order using REST api.payment method works fine with all online orders and i am using given api.
 
Request- (Put)

 http://MyHost.com/rest/V1/carts/67/order 
 json-{ "paymentMethod": { "method": "free"}}

Response-

 {
 "message": "The requested Payment Method is not available.",
 "trace": "#0 /chroot/home/mxwareho/wrenchbox.us/html/vendor/magento/module-quote/Model/QuoteManagement.php(337): Magento\\Quote\\Model\\Quote\\Payment->importData(Object(Magento\\Framework\\DataObject))\n#1 [internal function]: Magento\\Quote\\Model\\QuoteManagement->placeOrder(67, Object(Magento\\Quote\\Model\\Quote\\Payment))\n#2 /chroot/home/mxwareho/wrenchbox.us/html/vendor/magento/module-webapi/Controller/Rest.php(307): call_user_func_array(Array, Array)\n#3 /chroot/home/mxwareho/wrenchbox.us/html/vendor/magento/module-webapi/Controller/Rest.php(216): Magento\\Webapi\\Controller\\Rest->processApiRequest()\n#4 /chroot/home/mxwareho/wrenchbox.us/html/var/generation/Magento/Webapi/Controller/Rest/Interceptor.php(37): Magento\\Webapi\\Controller\\Rest->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#5 /chroot/home/mxwareho/wrenchbox.us/html/vendor/magento/framework/App/Http.php(135): Magento\\Webapi\\Controller\\Rest\\Interceptor->dispatch(Object(Magento\\Framework\\App\\Request\\Http))\n#6 /chroot/home/mxwareho/wrenchbox.us/html/vendor/magento/framework/App/Bootstrap.php(258): Magento\\Framework\\App\\Http->launch()\n#7 /chroot/home/mxwareho/wrenchbox.us/html/index.php(39): Magento\\Framework\\App\\Bootstrap->run(Object(Magento\\Framework\\App\\Http))\n#8 {main}"
 }