Hello Ottolien,
Thank you for using Apple Support Communities! I see that you haven't received an answer to your question. If you are able to provide additional information, then perhaps someone will be able to assist you. Some questions that might be helpful to answer are:
1. Are you manually entering your weight directly in the Health app or using a third-party app?
2. What happens when you try to enter your weight with a decimal? For instance: is the option to tap Add grayed out, you receive an error, or something else?
Cheers!