The ststm32 platform version 15.2.0 , being from January 2022, may have fallen prey to the auto-cleanup (=delete) registry process per Missing packages in registry - #4 by ivankravets. See currently available versions.
You may still be able to get it to work by referencing the Git download link
platform = https://github.com/platformio/platform-ststm32/archive/refs/tags/v15.2.0.zip
(provided no other packages it references aren’t also gone).
The other possibility would be to upgrade to the most recent ststm32 platform version (17.3.0), which should bring bugfixes (but possible API breakages) with it.
(FYI @ivankravets)