Does anyone know an extension to use for mac that can create faces and a tool that can close edges?
In the extension warehouse, check Eneroth’s page (bottom of the home screen).
She has a free extension names face maker or make faces.
just select lines, click on the adequate menu, it’ll fill all that it can.
there are some, I think one of fredo6’s does that. cleanup too maybe ? I’ve never really had this issue, not enough to get an extension at least, but I guess anyone working excursively with badly drawn DWGs will probably come around with an advice 
Try my extensions
s4u Make Face
https://extensions.sketchup.com/extension/440d2795-e729-42aa-81a2-2ba7df92ecc9/s4u-make-face
s4u Find gap or s4u Connect
https://extensions.sketchup.com/extension/888de95d-2d24-450f-86bf-7c0313daab3d/s4u-find-gap
https://extensions.sketchup.com/extension/3bca2b86-0f04-4c56-81a9-be9ab6717438/s4u-connect
I use the SKB Face tool to create faces, and find middle points, and edge tools to close gaps on lines, both are free.