/usr/local/nginx/logs/error.log
2022/08/11 09:36:47 [crit] 2821940#0: *6263 open() "/usr/local/nginx/proxy_temp/2/29/0000000292" failed 
(13: Permission denied) while reading upstream, client: 15.60.178.88, server: demo.jichu.com, 
request: "GET /static/css/bootstrap.min.css.map HTTP/1.1", 
upstream: "http://15.60.178.88:3001/static/css/bootstrap.min.css.map", host: "demo.jichu.com"
2022/08/11 09:36:49 [crit] 2821940#0: *6261 open() "/usr/local/nginx/proxy_temp/3/29/0000000293" failed (
13: Permission denied) while reading upstream, client: 15.60.178.88, server: demo.jichu.com, 
request: "GET /class/studChoiceConfig?v=463&_=1660181808733 HTTP/1.1", 
upstream: "http://15.60.178.88:3001/class/stu?v=463&_=1660181808733", host: "demo.jichu.com", 
referrer: "http://demo.jichu.com/class/class"

Solution:
vi /usr/local/nginx/conf/nginx.conf
#use nobody;
Modify to
use root;
Restart nginx