From the wiki:
Requirements
In order to start the collab scenario you need to have completed the main story of Part 2 (
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) and a few particular sidequests. These are:
-
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's sidequest (Saki)
-
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's sidequest (Four Shrines)
-
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's sidequest (Civil War)
- Recruit
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Only after doing all these will you be allowed to start.