Occasionally when you select a Form in Power Automate the drop down does not show the form and you need to find the FormID and manually enter it.
Firstly go back to your form and copy the FormID from the address bar. (Everything after the = sign)
Now go back to your Power Automate and select Enter custom value and paste the FormID
You also need to paste the FormID for Get response details.
Now you can select fields from your form as you normally would.