error is:
semantic error: while resolving probe point: identifier 'process' at :5:7
source: probe process("/usr/sbin/nginx").function("ngx_process_events_and_timers"),
^

semantic error: no match

Pass 2: analysis failed. [man error::pass2]
Number of similar error messages suppressed: 1.
Rerun with -v to see them.

and but this command work correctly
stap -e 'probe begin { printf("Hello, World!\n"); exit() }'
stap -e 'probe kernel.function("sys_open") {log("hello world") exit()}'