You can download and use apps that say in-app purchases freely as long as the app is free to use. Some apps do not open up till you subscribe, some open part of the functionality, some will not allow you to save your work, some will disable advance feature till you subscribe.
If you accidentally subscribe, you can also request a refund for a recent renewal of an in-app subscription purchaseÂ
Are you part of a Family Sharing?
With the limited information that you have provided, It is hard to even guess the problems faced by you.Â
You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.