I need to create a Bundle product and I need to be able to add a text field where my clients can add a personalized text that will be included in the selected product, but Magento will not let me add it, how can I do it?
As per understanding your question like you need to add a text area with the bundle product and the customers can add their custom text with that product right..?
Solution : Add Bundle product from magento2 admin panel and while filling up your info to the products area, you can find the Custom option tab click on it and choose the type as text area then save the product.
Result : Now you can see the result on front end with the custom field text area.
Path : Click On Catalog -> Product - > Add Product [Choose Bundle Product from list] -> Set dynamic pricing to No -> Chose Customizable Options Tab -> Click Add Option -> Give the Option Title and Chose the Option type as Text area, if you want you can give characters size too. Then save the Product. Check you front end.