The issue you are facing happens because the response from your form is being treated as a single string (not an array) or improperly formatted JSON, which causes the join function to fail, try below steps.
1. After the Get Response Details action in your flow, add a Compose action.
2. In the Compose action, write this code:
json(outputs('Get_response_details')?['body/FieldName'])
Replace FieldName with the name of your multiple-choice question.
3. Add another Compose action below this one.
4. In the second Compose action, write this code:
join(outputs('Compose'), ', ')
5. Use the output of the second Compose action wherever you want to place the multiple-choice question in your Word template.
6. If you have more multiple-choice questions, repeat these steps for each one.