Already discussed in Compilation failure using platform-ststm32 10.0.0. The JSON is wrong for the latest-greatest STM32 platform version.
Since your chip is STM32F030C8T6 and considering the available startup files in C:\Users\<user>\.platformio\packages\framework-stm32cubef0\Drivers\CMSIS\Device\ST\STM32F0xx\Source\Templates\gcc, one can conclude that you need the startup_stm32f030x8.s file and thus you must add
"product_line": "STM32F030X8",
e.g. as an attribute below the e.g. mcu field.
Is the NRST of the STLink connected to the target chip? Seems like a problem with either the reset line connection, or how openOCD is instructed to do the reset. With STM32CubeProg, does it work with software reset, hardware reset or core reset?