Triangles created and can not be 'healed'

roof has been made but a line on one surface appears and when I try and ‘heal’ by erasing, both triangles are erased

You have a co-planer problem. Simplest solution is to use Eraser Tool + Ctrl to hide that line.

An image would help…

First thoughts are that some of your eave lines or ridge lines aren’t parallel, so then SketchUp resolves it by ‘folding’ the expected-to-be rectangular roof surface into two triangles.
Because these two faces are not coplanar you cannot erase that extra diagonal edge.
Either hide it [Erase+Shift], soften it [Erase+Ctrl] or delete some of the problem geometry forming the triangles, and remake it differently, so that no ‘fold’ is needed…

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