How can I find where lines are open why I cannot make individual faces of it?
This extension is for you. Edge tools from Thom Thom.
https://extensions.sketchup.com/extension/52ab7474-469f-4a62-baa0-63c2b6390373/edge-tools
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The much more tedious way without extensions is to draw temporary edges across the face and see whether they split it.
Those dark edges are probably profiles (easy to check by toggling them off and on in the style), which are also clues to where there are problems closing the face.
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