STM32YYXX chip series is not defined in boards.txt
Overview

Following the guide Getting Started on the STM32 page on Github: Arduino core support for STM32 based boards:


  This repo is available as a package usable with Arduino Boards
  Manager.
  
  Use this link in the "Additional Boards Managers URLs" field:
  
  https://github.com/stm32duino/BoardManagerFiles/raw/master/STM32/package_stm_index.json
  
  For full instructions on using the "Boards Manager", see the Getting
  Started page.


Thanks to Juraj's comment for the tip 


  this is not the current STM boards package. ST Microelectronic took over the development github.com/stm32duino/Arduino_Core_STM32 stm32duino.com/viewtopic.php?f=16&amp;t=4671


Step by Step

The process is as follows (screenshots from OS X, but on Windows/Linux the process is the same):

Adding the URL in the preferences dialog (one URL per line)


  


Boards Manager menu


  


Boards Manager dialog


  


Boards Manager dialog - STM32 Core Boards installed


  


Boards Manager dialog - Boards installing


  


Boards Manager dialog - Boards installed


  


Here are the newly installed boards in the menu


  


As a comparison, these are the boards installed using the deprecated manual process described below:


  




Deprecated method


From STM32 - installation


  Prerequisites
  
  If your STM32 board does not have a bootloader yet and you want to be able to program it over USB, you'll have to upload
  the STM32duino bootloader to flash memory. See Burning the
  bootloader.
  
  Arduino and Arduino_STM32
  
  
  Download and install the official Arduino IDE. Versions 1.6.10 till 1.8.5 have been reported to work fine.
  
  
  If you already have the Arduino IDE installed and it's a newer version, see the Boards Manager package method of installation.
  
  
  Note: this method is deprecated and not supported in forum!
  
  
  Run Arduino IDE, and open the menu Tools -> Boards -> Boards Manager. Install the "Arduino SAM Boards" package (includes the
  Arduino Due) from the list of available boards. This installs compiler
  support for ARM Cortex-M3.
  Download a zip file containing the Arduino STM32 files from here.
  Unzip the content of the zip file, and place the Arduino_STM32 folder to [Arduino sketches folder]/[hardware]/[Arduino_STM32].
  Create the 'hardware' folder if it does not exist yet.
  
  
  Example how the path should look like:
  C:\Users\&lt;user&gt;\Documents\Arduino\hardware\Arduino_STM32




As an aside, for anyone using OS X, from Installation on OSX


  Installation on OSX
  
  
  Arduino_STM32 folder must be placed inside
  ~/Documents/Arduino/hardware (note. if the hardware folder doesn't
  exist you will need to create it) 
  So you should get ~/Documents/Arduino/hardware/Arduino_STM32
  Note.  DFU util binaries
  have now been added to the repo, in tools/macosx/dfu-util, so there is
  no longer a need to install Homebrew to then install dfu-util, or to
  compile dfu-util from source.