You have some duplicate faces that got creates with Surface-Knit2. I am not sure what that feature was for, but it isn't necessary. Perhaps you were trying to fix the the blue line that remained along the edge of the part after Surface-Knit1. This can be fixed by editing Surface-Knit1 and checking the box to close the gap listed.
If you end up deleting Surface-Knit2 you will have to fix all of your trims.
Anyway, I am not clear on what you are trying to knit to form a solid. You don't have an enclosed area. If you roll forward to include Surface-Plane7 then the box is enclosed, is that what you are trying to make solid? I don't see the purpose of the trimmed surface, except maybe to close up that little circular hole. If that was your intent then you could have done a simple Untrim command with the hole edge as your selection to close it up. If you do that and get rid of your Surface-Untrim2 and all of your trim features, then include the Surface-Plane7, then you are a lot closer to being able to form a solid.
The only thing left is a big gap in one corner of the model.
If you patch that up you should be able to form a solid.
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