For the first time build the app, you have to configure the Android package in app.json file.

If not it will return this error :

E:\Project\bytabookfinder>expo build:android
Your project must have an Android package set in app.json
See https://docs.expo.io/versions/latest/distribution/building-standalone-apps/#2-configure-appjson

To resolve this issue add this line of code in your app.json

"android": {
"versionCode":6,
"package": "com.perghstudio.bytabookfinder"
},

you can add before the Ios properties. You can change your package name.