Can't load xxxx.aux.mix file

I’m tring to load my own aux mixer file but always fail.
The reason I found was that in the startup file rcS, the program always went through the following branch and set SYS_USE_IO to 0, which caused the subsequent aux file loading process to be skipped.

I guess the problem is caused by hardware. The board I use is cuav_v5_nano.

How can I solve this problem?