Re: AI Builder FX doesn't work in the latest power apps
First of all, make sure to add the Sentiment Analysis model as a datasource.

Then store the result in your variable.
Set(Result, 'Sentiment analysis'.Predict(TextInput1.Text))
To get the sentiment, you have several options:
First(Result.Sentences).TopSentiment.Name
1. This will get the main sentiment of the first sentence.
Result.Document.TopSentiment.Name
2. This will get the main sentiment of the entire text.
See more details here https://learn.microsoft.com/ai-builder/prebuilt-sentiment-analysis