Hello,
The lock() and unlock() APIs exist for SPI framework that can sit on top of SCI SPI or RSPI driver. The framework allows you to manage multiple slaves in much easier way, by automatically asserting SSL on writes/reads and protecting the driver for prolonged access. When using single master+slave combination, my best suggestion is to disconnect the SSL pin from the peripheral (on the pins tab in the configurator) and use it in GPIO mode, manually asserting it before the write and releasing it thereafter.
For transfers longer than one word, the callback is only executed once (at the end of the transmission), provided no errors occurred. To observe callback after each byte, you'll need to disable DTC by removing it from the SPI configuration.
Simple PMOD display example for DK-S1 (available on Renesas website) illustrates how to use the SPI with single slave.
Regards