Check the permissions of the Form and ensure the group 'Guests' is enabled

Thats pretty much my issue. I have no idea how to access the forms permissions.

Click the form in edit mode and instead of clicking edit, click permissions.
Although if you are on basic permissions, you may not have that option.
Worth a look though.
P.S. This could also be a bug?

at this point i'm assuming your talking about the form block. Your correct that that is not available with basic permissions but I have turned on Advanced and everything is authorized for all users with nothing related to submission of the form. Clearly from the error the code expects some kind of permission set to allow submission.

could be a bug but i would think I cant be the first to submit forms from unregistered users.

Looks like it is a bug. After testing I have found it only effects Form blocks that use a pre-created express form. if I define the form in the block it works fine.

Finally tracked down the permission settings.

You have to go to the dashboard/system & settings/express/custom entry locations
there you need to click on the folder of your express entity and choose permissions from the popup.
All permissions are defaulted to only administrator.

Thanks for posting solution TRRosen!

Wondering what the logic is for making it default administrators, does not make sense, especially for using express forms for things like contact forms etc

In the event that someone else comes across this, detailed have changed since this discussion was posted.

Permissions are located here...

dashboard/system/express/entries

You can access this in the dashboard of your site using the URL

https://YOURSITEURL.com/index.php/dashboard/system/express/entries...

/hth