Development tools for Zephyr on STM32Workbench for Zephyr: VS Code extension to set up, build, flash and debug Zephyr applications on STM32.Devicetree manager: browse, edit and visualize Zephyr devicetree, STM32CubeMX-like for Zephyr.
Run the ST-Link Utility and verify that the target communication mode is set to "Connect Under Reset". My understanding is that if this option is not set for a L family device then the ST-Link can't communicate with the target device.