In iMIS EMS, there are a number of ways to generate an invoice, depending on what you are trying to accomplish.
Membership dues
To generate dues invoices, see:
Commerce orders
If the setting Order stage if order does not contain inventory items - Standard order types only (Settings > Commerce > General) is set to Completed, then orders containing non-inventory items are automatically invoiced.
If the setting is set to Ready for invoice printing, or the order contains inventory items, staff users must process the order through the order stages, which includes generating the order invoice. See Processing orders.
Event registration
If the setting Automatically generate invoices for registrations and adjustments upon checkout is enabled (Settings > Events > General), then event invoices are generated upon checkout. If this setting is disabled, you must manually generate invoices. See Event invoices.
Note: If you are trying to create the event invoice for the registrant to pay later, see Creating an event invoice for someone else to pay later.
Fundraising donations
When a donor submits their donation, an invoice is automatically created (Finance > Invoices). Staff can enter donations on behalf of donors that also generates an invoice once submitted.
If your association issues receipts to donors, see Issuing and viewing charitable receipts.
Other
To generate invoices without payments, you can choose to Pay Later, which generates an open invoice on the contact account page. See Using Pay Later.