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Yesterday I opened the Android project on another computer (files were on my onedrive, so I never 'moved' the files') to show the project to someone.

Now I reopened the project on my actual computer and lots of errors are generated (see image). The project worked fine before.

They were both windows computers.
I tried resyncing the gradle and rebuilding the project but the issue persists. What did I do wrong and more importantly: how to fix it (I'm very new to Android).

The strange thing is that I can still run the app without errors.

Any suggestions?

You are missing dependencies.

From the looks of it, you are missing Android Support dependencies and
Volley

Confirm your build.gradle has the appropriate library references and repos to pull it and that the new machine has the sdk installed as well.

If you are using pre-28 I would expect to see in your parent project gradle.

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript 
 repositories 
 google()
 jcenter()
 

 dependencies 
 classpath 'com.android.tools.build:gradle:3.2.1'
 classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.2.71' //if using kotlin
 classpath 'com.google.gms:google-services:4.1.0' //if using google services
 


allprojects 
 repositories 
 google()
 jcenter()
 mavenCentral()
 maven url 'https://maven.google.com/' //older android studio
 maven url "https://jitpack.io" //post api 28 needed
 


task clean(type: Delete) 
 delete rootProject.buildDir

Then in your app level build.gradle you should have dependencies to get your volley and support libraries like the following example if you are post API 28:
NOTE* If you are pre-28 then update the namespaces to non-androix pathways.

dependencies 
 implementation 'com.google.firebase:firebase-crash:16.2.1' //crash reporting

 //to compile @synchronized things and java references
 implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8:1.3.0"

 //Android ui support
 implementation 'com.google.android.material:material:1.0.0'
 implementation 'androidx.appcompat:appcompat:1.0.2'
 implementation 'androidx.legacy:legacy-support-v4:1.0.0'
 implementation 'androidx.cardview:cardview:1.0.0'
 implementation 'androidx.recyclerview:recyclerview:1.0.0'
 implementation 'androidx.constraintlayout:constraintlayout:2.0.0-alpha2'
 //Networking
 implementation 'com.android.volley:volley:1.1.1'

 // [START firebase]
 implementation 'com.google.firebase:firebase-core:16.0.5'
 implementation 'com.google.firebase:firebase-messaging:17.3.4'
 implementation 'com.google.firebase:firebase-appindexing:16.0.2'
 implementation 'com.google.android.gms:play-services-base:15.0.2'
 // [END firebase]

 implementation 'com.crashlytics.sdk.android:crashlytics:2.9.6'

Then whenever you open on a new machine just check that you have the SDK path accurate. For example the local.properties should never be checked into the repo as it has a hard coded path for your "local" properties.

Next make sure your SDK path is good in the menu, it's the folder with the 3 little blue squares:

If all of these are good, then do a build.clean and rebuild to see if that clears you up. If you still have issues you can try

File->Invalidate Cache and Reset

Lastly, if that still doesn't work, you may have build cache issues, so delete all your iml files, idea files, build folders and import the project fresh from code only. I'm guessing you are copying the physical folder of the project from one computer to another which would cause issues.

Hope that helps.

Happy Coding.