I will explain my question with more Details. In the mission Haven's Last stand, the Sin player can summon the Avatar after the second completed mission. In my opinion the Avatar can only be summoned on a space with a Nest. In the let's play video from cmon the avatar was summoned on a space without a Nest token and I try to understand why this is possible.
If by that video you mean (https://youtu.be/uQxlskCen2I?t=3285), this may be due to the prototype status of the game during the video shooting.
The rules as printed are clear in that regard:
If an event calls for a Monster to be summoned onto the board, the normal summoning rules are followed (see Summon Monsters on page 16). However,
in this case, if all Nest tokens are blocked by Monsters,
the new figure is placed on any Nest token on the board
Note that "Nest token" refers to printed tokens as well as physical tokens.