I got this Flatpickr error on an ASP.net project because a new connection to Jquery was automatically create in the application, so it looks as though this is created by a conflict. Because of the page flow construct jQuery.noConflict() did not work when using web form validation; a combination of asp.net 4.5 Web Forms Unobtrusive Validation jQuery Issue and this <asp:ScriptManager in Asp.Net webforms, how it works? should have worked, but only a partial success. Although not directly relevant to anyone not using the Microsoft stack, it is relevant to people using Flatpickr this may allow someone to understand what is going on and where the error "$('.selector').Flatpickr is not a function" is occurring and find a solution in their project idiom.