When unauthenticated users try to submit a public catalog item in Service Portal, a 400 Bad Request error occurs and they are unable to submit. This issue is due to AngularProcessor authentication restrictions.
Steps to Reproduce
Proceed to any catalog item.
For example, an iPhone 6S (https://<instance-name>.service-now.com/sp?id=sc_cat_item&sys_id=d0b15e33d7033100a9ad1e173e24d49e) and make it public.
For more information, see the product documentation topic Service catalog items.
Go to the Service Portal page (sc_cat_item) and make this page public.
Make any widgets associated with the sc_cat_item page public also.
In an incognito window, go to https://<instancename>.service-now.com/sp?id=sc_cat_item&sys_id=d0b15e33d7033100a9ad1e173e24d49e.
Note that although the catalog Item is public, it will not allow you to submit the item.
Related Problem: PRB854474