Can anyone explain why I can't use the fill pattern feature on this part?
Maybe... source object is to big?
I don't know why, but setting Margins to 1mm stops it. Set it to zero and the pattern fills:
I also created a new sketch on the face and did a convert entities on the boundary.
If you need the 1mm margin, create a new sketch offset 1mm from the original boundary and use this for the fill.
EDIT: Why this is failing with a 1mm margin is probably due to this new boundary most likely crosses itself in some of the tighter places.
Create a 1 new sketch offset 1mm from the original.
Inspect the new sketch along its entire length looking for crossed or touching sections.
Trim away any such found sections as most likely, the fill won't go into them. If they are large enough for a potential fill, trim such that there become a separate contour in the sketch (or as another sketch added to the Fill boundary list, if possible).
Fill using this sketch as a boundary with zero margin.
Since your shoreline of Pangea is composed of two entities, when a margin is added, gaps may be created, building an open boundary and causing the original fill boundary to fail.
When you create the 1mm offset sketch, check these areas for gaps and if open, close them. Or join the two pieces together with no endpoint.
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