Unfortunately, FreeRTOS does not save the stack size of a task. Therefore, the debugger doesn't know the end of the stack and cannot calculate the stack coverage.
If all tasks use the same stack size, you can specify the stack size in bytes as the second parameter to the TASK.CONFIG command. E.g. for Arm based systems with each task having a stack of 1024 bytes:

TASK.CONFIG ~~/demo/arm/kernel/freertos/freertos.t32 0 0x400

Refer for more information to the chapter "Manual Configuration" in OS Awareness Manual FreeRTOS.