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 CRM > Invoices > + New Invoice.
To create Invoices using the App, go to Accounts > Invoices > + symbol in bottom right corner (to create a 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> Contacts > edit arrow for contact > New > Invoice.)
Web Portal Dashboard, Quick Action: