Though the message says polygon, the infill plates feature works only with 3-sided and 4-sided boundaries. That is because, the plate elements being created can only be 3-sided or 4-sided.
For example, if you choose the sides of the polygon that looks like a hexagon, and attempt to create a plate using the Infill option, you will encounter the message you stated in your question.
If the boundary contains more than 4 sides, add dummy members diagonally between nodes so that it is reduced to a combination of 3 and 4 sided units, then select all members that are associated with that region, and go to Infill. For example, if the hexagon is between nodes A-B-C-D-E-F, in addition to members joining AB, BC, CD, DE, EF and FA, you will also need members like AC and DF that reduces the hexagon to 2 triangles and 1 rectangle.
If it is a quad plate, make sure all 4 nodes lie in one plane. Else, you will end up with a warped element that can produce inaccurate results.
If you are still unable to make it work, please post your model and let us know which polygon is causing trouble.
For example, if you choose the sides of the polygon that looks like a hexagon, and attempt to create a plate using the Infill option, you will encounter the message you stated in your question.
If the boundary contains more than 4 sides, add dummy members diagonally between nodes so that it is reduced to a combination of 3 and 4 sided units, then select all members that are associated with that region, and go to Infill. For example, if the hexagon is between nodes A-B-C-D-E-F, in addition to members joining AB, BC, CD, DE, EF and FA, you will also need members like AC and DF that reduces the hexagon to 2 triangles and 1 rectangle.
If it is a quad plate, make sure all 4 nodes lie in one plane. Else, you will end up with a warped element that can produce inaccurate results.
If you are still unable to make it work, please post your model and let us know which polygon is causing trouble.