You have two options:

Option 1: Upgrade Gradle (Recommended)

Since Java 21 is installed, update Gradle to a compatible version (8.5 or later).

Open your android/gradle/wrapper/gradle-wrapper.properties file.

Change this line:

distributionUrl=https://services.gradle.org/distributions/gradle-7.6.3-all.zip

To:

distributionUrl=https://services.gradle.org/distributions/gradle-8.5-all.zip

Open android/build.gradle and update:

classpath 'com.android.tools.build:gradle:8.1.0'

To:

classpath 'com.android.tools.build:gradle:8.3.0'

Clean and rebuild the project:

flutter clean flutter pub get

Option 2: Downgrade Java

If you want to keep Gradle 7.6.3, downgrade Java to version 17:

Install Java 17

Restart the terminal and run:

flutter doctor --verbose