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.
You probably have created a new project with "No firmware" selected. The main.c is not provided. To me, this error means you don't have the main function or it cannot find it. Verify your sources file are under the "src" folder.