Cannot fit requested classes in a single dex file (# methods: 86261 65536) com.android.builder.dexing.DexArchiveMergerException: Error while merging dex archives: The number of method references in a .dex file cannot exceed 64K.

defaultConfig {
    ...

    multiDexEnabled true
}

defaultConfig {
    ...

    multiDexEnabled true
}

android {
    defaultConfig {
        ...
        minSdkVersion 21 <----- *here
        targetSdkVersion 26
        multiDexEnabled true <------ *here
    }
    ...
}