I found a solution here:
https://forums.freertos.org/t/lpc55s69-evk-solid-tf-m-non-secure-freertos-example-search/19631/5
Hello @AlexKuznetsov
The purpose of dividing Secure and non-secure is to protect secure part from being read by non-secure ones. So why do you want to call secure function on secure world?
There is freeRTOS secure/non-secure demo under SDK:
BR
Alice
This is a different stuff.
I would say your example is a ported solution from FreeRTOS (https://www.freertos.org/RTOS-Cortex-M33-LPC55S69-MCUXpresso-GCC.html)
In the NXP SDK you have tfm based examples like tfm_demo_ns & tfm_demo_s, unfortunately the NS part is a bare-metal example.
So my main problem is a non-secure FreeRTOS part and I'm still looking for a such example.