This issue was discussed in this forum topic: Can't select accounting basis in reports. I thought it had been resolved, but apparently not, so I will raise it again with the developer through some moderator magic.
Meanwhile, if you create a sales or purchase invoice, the option should appear, even if you immediately delete the invoice.
Reports do not appear when they would be out of context. The Customers tab must be enabled and you must have at least one customer and sales invoice entered. Otherwise, there is no purpose for a customer statement.