With the correct permissions, Invoices can be created in both the Web Portal and the App.
To create Invoices using the Web Portal, go to Accounts > New Invoice. (Note: You can also select + Invoice from the dashboard.)
You can also go to Accounts > Invoices > + New Invoice.
Invoices can also be created through a job, in the CRM contact itself and on the Web Portal Dashboard through Quick Actions Add New icon.
CRM Contact Activity: (Go to CRM > All Contacts > edit arrow for contact > New > Invoice.
To create Invoices using the App, go to Accounts > Invoices > + symbol in bottom right corner (to create a new invoice)