I have found a way around this issue, is not the perfect solution but it might work for you, depending on your game.

I have had the same issues when trying to upload to Google Play. and getting no.OBB Key Found.

My. APK file was 45MB and the.OBB 125MB After discovering that Google Play has an application size of 100MB, I thought that maybe the error was coming because my .apk + .obb file was exceeding 100Mb

This is how I solved this issue

I had to go into my game and optimize every single texture I could, and deleting unnecessary animations, audio, and other assets. I have used Texture size : 256px X 256px and for phone I think it looks great.

I also had to check the settings Package game data inside the .apk? (true) I was able to export a 99.6MB .apk file and then I uploaded it to Google Play.This had fixed the OBB Key Not Found for me. I was able to download the game from the store and play it perfectly.

Other Settings to help with Optimization

Package Settings-> Create Compressed cooked packages (true) Android-> Force small OBB Files (true)

I have tried exporting the game without the game data inside the .apk and even both files not exceeding 100MB I could not make it work on Google Play. Therefore I would recommend having the settings,

Package game data inside the .apk? (true)

If anyone finds a better alternative please share here, as I think this error could be from something else. Just sharing what has worked for me.