Hi swm023,
To answer your first question, regarding the error message you are receiving while creating a Work Order within the "Scheduling and Invoicing" app, this is likely something we we need to see first hand to help resolve. Would you be able to send me a private email with the Company Account name you use to login to your Method Account with? Also, please make sure MethodSupport is turned on, go to Customize-Users, and scroll down until you find a user called "MethodSupport". Make sure there is a checkmark for this user in the "Active" column.
In regards to your second question, essentially asking if it is possible to tweak Method to work for your business' workflow, and if you can create a button that would create a Work Order when closing an oppotunity. This would definitely be possible, and shouldn't be too difficult to perform, however this depends on your Customization skills and if you plan on doing this yourself. If so, I would follow the Method Webinar videos on Customization, here is a link, these videos should help you familiarze yourself with customizing in Method. You could also reach out to a Method Consultant, they would be able to discuss what changes you are looking for, and help customize your account accordingly. If you do want to considering contacting a Method Consulting, you can get more information from this link.
In regards to your last question about how feasible it would be to add a documents link to the add/edit Workorder screen, this is a little difficult to answer. This would likely be possbile, but it would really be your decision if this is required here. If you decide to speak with a Method Consultant regarding an automated Work Order button, this could also be a good question to ask them. The Consultant would have a better idea about your business work flow, and could tell you if this is worth while, and how they would go about doing this.
-Ben