"expected primary-expression before '__attribute__'",

This issue has already been discussed at Fastled clockless_trinket.h:326:24: error - #6 by maxgerhardt. You need to use a newer compiler version to use this library. Adding

platform_packages =
  ; use GCC AVR 7.3.0+
  toolchain-atmelavr@>=1.70300.0

to the platformio.ini fixes it.

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