I deployed Field Service 7.3.0.123 to two v9.0 organisations. In both cases I got a JavaScript error when opening up the main "Opportunity" form, for a new or existing Opportunity.

The reason is as follows:

On the main Opportunity form, FS.Opportunity.Library.Load has been added as an OnLoad event handler. However, the following scripts were not added to the form's Form Library list:

msdyn_/Utils/head.js
msdyn_/Opportunity/Opportunity.Library.js
msdyn_/Utils/ResourceLoader.js

The event handler and the three libraries were added to a new "Field Service Information" form that got deployed as part of Field Service. Adding the libraries to the main form resolves the issue.

Cheers,
John