build:server & export:server commands are not working

[skptricks]

getting below error message while executing below commads :
yarn build:server
yarn export:server

D:\sumitprojects\NextSimpleStarter>yarn export
yarn run v1.17.3
$ npm run build:server && npm run export:server

next-simple-starter@3.0.0 build:server D:\sumitprojects\NextSimpleStarter
NEXT_TARGET=server next build

'NEXT_TARGET' is not recognized as an internal or external command,
operable program or batch file.
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! next-simple-starter@3.0.0 build:server: NEXT_TARGET=server next build
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the next-simple-starter@3.0.0 build:server script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! C:\Users\Acer-sumit\AppData\Roaming\npm-cache_logs\2019-08-26T19_52_15_967Z-debug.log
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.