Is there a way to create default templates to be added to the description section automatically on creating a story/task. I know JIRA provides a lot of add ons available in marketplace that provide similar features at a price, but I am looking for something free of cost.
JIRA automation will allow you to populate a field when a story/task is created.
You can use that on the description field, though it won't stricly be a template. You could perhaps insert the text you want using square brackets or something similar to indicate where the user needs to insert the right information.
For example:
Acceptance criteria: [insert acceptance criteria here]