[SalesForce] ReferenceError: require is not defined

Interesting - there are a few things going on here I think but let's first focus on the use of onclick= bound to a non aura client side controller - that should plain not work, ever. In aura markup what looks like a simple div is actually syntactic shorthand for something more like:

<aura:html tag="div" HTMLAttributes="{!bagOfAttributes}">

and in LC/Aura all event handlers must be bound to {!c.someHandler} expressions and not plain old JavaScript. If direct binding to a global plain old JavaScript function works I believe it's 100% unintentional and likely to be "fixed"/removed.

Second, this looks like LockerService's expected behavior inherited because all component JavaScript and any external JavaScript you require/import now runs with JavaScript strict mode enabled which cleans up many long standing global scope issues. In helper01.js are you publishing toggleSection() as a global variable? In other words simply declaring a function no longer makes it global - same is true for any variable/object in strict mode. You need to explicitly indicate that you want it added to global scope like so:

window.toggleSection = toggleSection;

You can of course directly assign window.toggleSection = function() { ... };

This is a strict mode standard and is not specific to LS.

Lastly, ltng:require is itself a Lightning component and is designed to be used inside of the component lifecycle - it's dynamically including your CSS and scripts much like RequireJS and other dynamic loaders do with a twist: it's integrated into the init/render flow. It removes most of the issues around dynamic loading (inherently asynchronous) and LC's component lifecycle but to get the benefits you'll need to work inside of the life cycle's constraints.

Rolling up comment as answer:

As per your original post, you had declared the event variable within the if block and you were using it outside the scope and thus you were getting undefined variable error.

if(state == "SUCCESS") {
        var sobjectEvent = $A.get("e.force:navigateToSObject"); // declared here
        sObjectEvent.setParams({
            "recordId": response.getReturnValue(),
            "slideDevName": "detail"
        });

    }
});
$A.enqueueAction(action);     
sObjectEvent.fire();  // used here and thus error

Later when you moved it to the if block, it was case sensitivity which was not working.

if(state == "SUCCESS") {
        var sobjectEvent = $A.get("e.force:navigateToSObject");
        sObjectEvent.setParams({
            "recordId": response.getReturnValue(),
            "slideDevName": "detail"
        });
        sObjectEvent.fire(); // it was case sensitivity this time, declaration needed to be changed to sObjectEvent (notice the upper O)                   
    }
});
$A.enqueueAction(action);   

Finally, changing the variable name to sObjectEvent and using it within the block where declared resolved the issue.