If you generated a poster that is mostly blank, this is because:
the person you selected as your starting person does not have ancestors in Ancestry.com or FamilySearch.org.
or
the individuals aren't properly connected within the Ancestry or FamilySearch family tree.
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, a parent, etc. The poster will include the starting person's ancestors. If you choose a starting person at the end of your line, the poster will not generate anything because you do not have ancestral information for that person.
If a blank poster is being generated, please go to your family tree on Ancestry.com or FamilySearch.org and check the profiles of the missing ancestors. If their children and/or parents are not listed in their 'Facts' profiles, please add them and try generating a new poster in MyCanvas.