Hi, this is my first post in kodular community. I am trying to help you about Android SDK ; because many people suffers from this problem (including me also) when posting the .aab on Play Store.
- Export the .apk file:
So, firstly what we have to do is to export the .apk of the file (we will not gonna export.aab from kodular because it will export the aab with their limited SDK … Which we are not discussing about)
- Export the keystore file :
Now you have to go to the builder where you have created the app, and tap on the corner button (account) and select “preferences” from the drop down list.
A pop up window will open. Now you can see the keystore SHA1. ( which you use to connect your project with firebase) now, tap on the “export keystore” button, and a file will be downloaded.
- Updating the SDK (API level change):
You have to download th apkTool M app (download links are provided at the end of this post)
And now open the apkTool m app and grant all the required permissions, then all the files and folders in your device will appear on the screen ; from where you can select the .apk file you have exported and then a drop down list will appear and from the list ; select “quick edit”.
Then a pop up window will appear. There you can modify the minimum SDK level.( I think you will not need this option because kodular already allows you to set the minimum SDK level)
So, just next to the minimum SDK level option, you will see the “target SDK” option, there you will select the latest API level (which you couldn’t do in the kodular) (e.g. if my apk in kodular is made in api level 30, I have to change it to 31 to post on playstore for now )
Then in the signature option click don’t sign (because you will sign it manually with your exported keystore file) and click “save”.
The saved .apk will appear on the files list.
- Convert .apk to .aab for upload:
Unfortunately, Google play console now doesn’t allow you to upload apk file, you have to make a .aab file for it. So, open the apk to aab converter app ( download links are provided at the end of this post)
And select the modified apk which you have made with newer api level on apkTool and convert it to .aab (it is super easy, you have to just select the apk and write the name of the aab file and click on the convert button, so simple)
- Add Signature to your.aab file:
Google does not allow you to upload.aab without signature, because keystore signature is your app’s identity which is very important. So you have to download the zip signer app ( download links are provided at the end of the post) and then open the app ; click on the text box “input file” and then tap on the “choose in/out” button and select the .aab file.
And now go to the corner three dots option and go to the option “my keys” and then a screen will open. From there again click on the corner three dots and select “register keystore” option and now select the keystore file (which you have exported on the step 2) and it will ask for a password; to open the keystore file , by default all the builders have set the password to “android” (all small characters, i.e. password is case sensitive) so you have to put this password and after that the keystore file will be imported.
Now go back to the page where you have already selected the import path of the .aab , there you will see the sign mode ; tap and select the option named “Androidkey” (if you have not imported the keystore file it will not show the option) and the click on “sign the file” option and the .aab will be signatured. And the signatured .aab file will appear ont he folder where the old .aab was.
Now, you are ready to go ! You can upload it to play store without any errors. 
Links 
:
Tutorial video (hindi)
Edit by mod, Please provide English speaking video.
apkTool M app :
Apk to aab converter:
Zip signer app :