Private pension contributions are tax-free up to certain limits. Usually, you can get a tax relief worth up to 100% of your annual earnings. Just ensure your provider is registered with HM Revenue and Customs (HMRC).
On the other hand, QuickBooks Self-Employed complies with the HMRC’s available SA103F Categories. However, the ability to categorise personal pension contribution is not yet available. What you can do for now is to categorise it as Personal Expense. For more detailed information, you may check this article: SA103F Categories.
Also, I’d recommend consulting with your accountant so you’ll be properly in categorising your personal contributions.
For more insights, you may check these articles:
Keep me posted if you have other questions. I’m always here to help.