ATTiny85 433Mhz with sleep
The error you posted has nothing to do with an Arduino core. This is just plain avr-libc stuff. In order to use this kind of functionality, you need to

#include &lt;avr/io.h&gt;


In the Arduino core, it is common to have this done by default. But if this is not the case with your chosen core, just add the #include yourself in the program.