Event not found in sed

I tried the code as given on delete text between curly brackets

however I am facing this different error regarding event in sed.

file contains:

This is {
{the multiline
text} file }
that wants
{ to {be
changed}
} anyway.

sed ':again;$!N;$!b again; s/{[^}]*}//g' file

what is supposively going wrong in the workout?