I am trying to export a Signed APK to submit to the play store but when I click Build->Generate Signed APK that option is grayed out and I can not select it.
I'm not sure why this is happening, but as a workaround you could use gradle build tasks.
I generally do all this from the command line. If you don't have any build flavors, try doing the following:
./gradlew assembleRelease
You can see multiple ways to sign your build using this process here: How to create a release signed apk file using Gradle?