node.js – nginx 502 badgateway shows on opening my hosted site developed using nodejs,also on lodding pm2 shows App is not defined

on logging pm2 error shows /home/ubuntu/.pm2/logs/plantly-error.log last 15 lines:

0|plantly  |     at Module._compile (node:internal/modules/cjs/loader:1364:14)
0|plantly  |     at Module._extensions..js (node:internal/modules/cjs/loader:1422:10)
0|plantly  |     at Module.load (node:internal/modules/cjs/loader:1203:32)
0|plantly  |     at Module._load (node:internal/modules/cjs/loader:1019:12)
0|plantly  | ReferenceError: app is not defined
0|plantly  |     at Object. (/home/ubuntu/index.js:1:1)
0|plantly  |     at Module._compile (node:internal/modules/cjs/loader:1364:14)
0|plantly  |     at Module._extensions..js (node:internal/modules/cjs/loader:1422:10)
0|plantly  |     at Module.load (node:internal/modules/cjs/loader:1203:32)
0|plantly  |     at Module._load (node:internal/modules/cjs/loader:1019:12)
0|plantly  |     at Object. (/usr/lib/node_modules/pm2/lib/ProcessContainerFork.js:33:23)
0|plantly  |     at Module._compile (node:internal/modules/cjs/loader:1364:14)
0|plantly  |     at Module._extensions..js (node:internal/modules/cjs/loader:1422:10)
0|plantly  |     at Module.load (node:internal/modules/cjs/loader:1203:32)
0|plantly  |     at Module._load (node:internal/modules/cjs/loader:1019:12)

how to solve

I was expecting as error free,due this when I ‘m trying to get my site,it shows 502 bad gateway,why