You must have "Full Access" or "Limited Access - Financials" permissions.
An invoice can only be voided if no payment has been made. If the invoice has a full or partial payment, the open payment(s) need to be "credited" down to $0 and will show as canceled.
From the left navigation menu, click Financials > Invoicing.
Find the desired invoice, and click the Sale ID.
In the top right corner, click the 3 dots > Void.
Verify by clicking Void.