Why doesn't the sei() function work?

Question

Atmel AVR

Given: A one-stop-shop php/c# jack-of-all-trades programmer, ATtiny13A (first attempt at avr) and problem:
If sei() is added to the code, it is underlined with a red squiggle on build as well the error ld returned 1 exit status TinyBlink collect2.exe takes off, and if this line is commented out, everything is fine.
what to do = ?
Thank you in advance!
PS The code was partially written by myself, and partially copied from here: https://geektimes.ru/post/255772/ and from many other sources about interrupts, shims, etc.

Answer 1

Forgot to include interrupt.h

Answer 2

when trying to figure it out, you need to figure it out, and not copy pieces of shit code from other authors into your shit code
https://forum.pjrc.com/threads/29172-ld-returned-1...