Will not let me install my apk in versions lower than android 7

Throughout the project, it was possible to install the app from Android studio in different versions of Android and there has never been a problem. When I generated the apk today, when installing it on phones whose Android version exceeds 7 there is no problem, but if the version is lower it does not let install. I have made sure that it allows for eternal applications and the instant run is deactivated

build.gradle Module:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 26
    buildToolsVersion '26.0.1'
    defaultConfig {
        applicationId "com.drinkme.sdm.myapplication"
        minSdkVersion 21
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner 
"android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 
'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    //noinspection GradleCompatible
    compile 'com.android.support:appcompat-v7:26.+'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    compile 'com.android.support:design:26.+'
    compile 'com.android.support:support-v4:26.+'
    compile 'android.arch.lifecycle:extensions:1.0.0-rc1'
    compile 'android.arch.persistence.room:runtime:1.0.0-rc1'
    testCompile 'junit:junit:4.12'
    annotationProcessor 'android.arch.lifecycle:compiler:1.0.0-rc1'
    annotationProcessor 'android.arch.persistence.room:compiler:1.0.0-rc1'
}

build.gradle Project:

buildscript {
    repositories {
        jcenter()
        maven {
            url 'https://maven.google.com/'
            name 'Google'
        }
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.3.3'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        jcenter()
        maven { url 'https://maven.google.com' }
        maven {
            url 'https://maven.google.com/'
            name 'Google'
        }
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

Manifest:     

    <application
        android:allowBackup="true"
        android:icon="@mipmap/mediano_sin_fondo"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/mediano_sin_fondo"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name=".LoginActivity"
            android:label="@string/title_activity_main">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".MainActivity"
            android:label="@string/title_activity_main"
            android:screenOrientation="portrait" />
        <activity android:name=".crearCuenta.CrearCuentaActivity" />
        <activity
            android:name=".PerfilActivity"
            android:label="Perfil"
            android:screenOrientation="portrait" />
        <activity
            android:name=".MasLogrosActivity"
            android:screenOrientation="portrait"
            android:parentActivityName=".MainActivity">
        </activity>
    </application>

</manifest>

If you need more information, ask for it. Thank you very much in advance.