When you are making your poster a standard poster or a combination poster the poster may come out blank. This is because
the person you selected as your starting person does not have ancestors in Ancestry.com
the individuals aren't properly connected within the family tree in Ancestry.com
When creating your poster, be sure to select a starting person who is closer to the beginning of your line. Someone like you, a child, parent, etc. These posters will include the starting person's ancestors. If you choose a starting person at the end of your line, the poster is not generating anything because you do not have ancestral information for that person.
If a blank poster is being generated, please go to your family tree in Ancestry.com and check the profiles of the Ancestors that are missing. If their children and/or parents are not listed on their profile's 'Facts', please add them and retry generating a new poster in MyCanvas.