Installed Android 4.1 on Ubuntu 20.04. Getting the following error:
Gradle sync failed: com.android.tools.idea.gradle.project.sync.idea.issues.SdkPlatformNotFoundException: Module: 'app' platform 'android-29' not found.
On opening the SDK manager I get the following screen:

I also executed
$ANDROID_HOME/cmdline-tools/tools/bin/sdkmanager --licenses
How to fix this?
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This is what shows in the SDK Platforms tab. Unchecking the "Hide Obsolete Packages" doesn't change anything.
go to the "sdk platforms" you can find it by click on navigate->find anywhere than enter "sdk" than what you need is to find the api level you need (29 in your case) and check the box near it.
As the error is saying. Install the SDK Platform with API Level 29. YOu need to switch to the tab " SDK Platforms and install the right API Level:
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maybe you changed accidently your SDK location.
Mine is on C:\Users\elmo\AppData\Local\Android\Sdk
Pay attention: using it on windows.