While exporting a PDF, an error occurs “Unable to create PDF” if your document contains a box which has linear blend applied to it and ‘Registration’ is used as one of the blend colors.

Create a new Registration color with the value of R=0% G=0% B=0% and use it in-place of the original ‘Registration’ color.
Following are the points to Create registration color:

1. Click (+) sign on the colors palette.
2. Name the new color
3. Select ‘RGB’ under the Model drop-down.
4. Enter values for R = 0% , G = 0% , B = 0%.
5. Click ‘OK’ button.