Sorry, this action is not available in simulation

According to the doc:

Turn on the Web & App Activity, Device Information, and Voice & Audio Activity permissions on the Activity controls page for your Google account. You need to do this to use the Actions Simulator, which lets you test your actions on the web without a hardware device.

And I had do what Jeremy Gordon suggested. To add a second google account in the GCP IAM console with a viewer action permission and then login with this second google account in an incognito window for the web simulator to work.

I did not get this message on my invocation, but on my my second input:"Sorry, this action is not available for your app.". It turns out the simulator had left the conversation right after the invocation (and it did mention that in the small print).
This happened because I returned a FinalResponse for the invocation. And a final response is pretty final, it will terminate your conversation.

So, after a FinalResponse you can only get back in your action/conversation by a new invocation or a deep link. If you want to suggest questions/inputs, then you should return ExpectedInputs.

You might need to turn on Web & App Activity to let group members use some Google Assistant features.(if you are using organizational account)

https://support.google.com/assistant/answer/7219584?hl=en

If you are using any organization's Google Accounts then there might be an access issue. So use your own personal Gmail account.

Take a look here, organization's might not be given you access. So use your personal Gmail and follow the Docs, you will able to create your agent/ actions and able to test it with simulator as well as in android device.

At time of testing the Google Action you need to set the location to the country which you selected while developing or submitting your Google Action. By default US is selected in testing but if your action is for one particular country only then you need to select that. You can see in image the field where you can select location