Select "Contributed" type, search for "STM32 MCU based boards" and install: 3.2 Selecting the STM32 microcontroller. After the installation is complete, close the board manager and go to Tools > Board > STM32 MCU based boards. A list of STM32 MCUs appears, so you can choose between Nucleo, Discovery and Evaluation boards, or generic STM32 MCUs.