Cannot Create/Edit Invoices - Can't find page requested

I am trying to send out monthly invoices to my clients. When I click to create a new invoice, whether it is through the general invoices page or clicking next to each customer to create an invoice, I reach a page that says “We're sorry, we can't find the page you requested.”

I have tried clearing my cache, I have tried an alternate browser, and I have tried searching for additional information on how to resolve this - all to no avail. If I cannot send invoices to my clients, I don’t know how I can use this platform to operate a business. 

Thank you so much for your swift attention to this matter.