non-manifold inputs

[riskgamer13]

Hello, I am recieving this message "the attempted shell operation has non-manifold inputs" and have looked at what it meant but still couldn't find where the issue might be at.  The drawing is for a school final though and if I can't figure out the problem area it will not be as complete as it is required.

[JDMather]

Drag the End of Part marker to just below Extrusion45.

Examine the part carefully (can you find the problem)?

Examine Extrusion 52 and 53. Can you see the problem?

Examine Extrusion 56 carefully.  Do you see the problem?

Sketch1 is not constrained or making use of obvious symmetry about the origin.
I recommend you read this and then start over. http://home.pct.edu/~jmather/SkillsUSA%20University.pdf

Also, have you installed all Service Packs?

[riskgamer13]

I saw Where some of the issues were and fixed them to the best of my ability, though i did not understand what you meant by "Sketch1 is not constrained or making use of obvious symmetry about the origin."  The computer i am using is a school computer so i assume that all the services are installed.