The action 'Post adaptive card and wait for a response' doesn't allow for adaptive card JSON to be entered, it only allows for a plain text message to be entered (similar to the action 'Post message in a chat or channel'). The similar action called 'Post adaptive card in a chat or channel' that doesn't wait for a response, seems to work fine.
Here's a screenshot of the action I'm having issues with, showing the option for JSON to be entered is missing:

I've achieved this in the past with the 'Post an Adaptive Card to a Teams channel and wait for a response' operation, however this has now been deprecated according to this doc Microsoft Teams - Connectors | Microsoft Docs
I've tried:
Enabling experimental features from the PA settings menu
Selecting Post Virtual Agents (Preview) from the 'Post as' dropdown of the action, but I do'nt have any bots to post as.
Select Flow bot from the 'Post as' dropdown of the action, and choosing a team, channel and typing a test message.
When testing the flow with the options that are available, the fails and returns this exception:
The request failed. Error code: 'InvalidJsonInBotAdaptiveCard'. Error Message: 'Microsoft.Azure.ProcessSimple.Data.Entities.Exceptions.ProcessSimpleDataException: The specified Teams flowbot adaptive card request is missing or invalid. The tracking Id is '{0}'. ---> Newtonsoft.Json.JsonReaderException: Error parsing boolean value. Path '', line 1, position 1.
at Newtonsoft.Json.JsonTextReader.ParseTrue()
at Newtonsoft.Json.JsonTextReader.ParseValue()
at Newtonsoft.Json.Linq.JObject.Load(JsonReader reader, JsonLoadSettings settings)
at Newtonsoft.Json.Linq.JObject.Parse(String json, JsonLoadSettings settings)
at Microsoft.Azure.ProcessSimple.Data.Components.AdaptiveCards.AdaptiveCardBuilder.DeserializeAdaptiveCard(String serializedAdaptiveCard, RequestIdentity requestor, CultureInfo cultureInfo, BotAdaptiveCardAtMentions atMentions)
--- End of inner exception stack trace ---
at Microsoft.Azure.ProcessSimple.Data.Components.AdaptiveCards.AdaptiveCardBuilder.DeserializeAdaptiveCard(String serializedAdaptiveCard, RequestIdentity requestor, CultureInfo cultureInfo, BotAdaptiveCardAtMentions atMentions)
at Microsoft.Azure.ProcessSimple.Web.Common.Validation.BotAdaptiveCard(String card, RequestIdentity senderRequestIdentity)'.
Am I going mad, or does this not work for other people?