Too many classes in --main-dex-list main dex capacity exceeded

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I’m getting this error even if the multi dex option is enabled in gradle.
Any idea why it is happening and how to resolve it?

UNEXPECTED TOP-LEVEL EXCEPTION:
com.android.dex.DexException: Too many classes in --main-dex-list, main dex capacity exceeded
at com.android.dx.command.dexer.Main.processAllFiles(Main.java:494)
at com.android.dx.command.dexer.Main.runMultiDex(Main.java:334)
at com.android.dx.command.dexer.Main.run(Main.java:244)
at com.android.dx.command.dexer.Main.main(Main.java:215)
at com.android.dx.command.Main.main(Main.java:106)