Adobe Community Professional, Feb 23, 2021Feb 23, 2021
John Mensinger • Adobe Community Professional, Feb 23, 2021Feb 23, 2021
An InDesign document exists in a "Transparency Blend Space," assigned according to the intent of the document when it was first new (simply: Print = CMYK, Web = RGB).
It's on the Edit menu:
When you export to PDF, the Output > Color setting targets a specific "Desitination" color space:
When those two settings don't match, you get the error.