Hi @Maximiliano_Gibert and welcome to the Losant Forums.
Based on your screenshot, my best guess is that device attribute has a trailing space at the end of it (for example, "humidity " instead of “humidity”). Can you see if that is the issue?
If not, can you send me the ID of the device where you are trying to add this attribute?
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Thank you very much! Yes, I fixed it up by deleting an end space, apparently its a normal beginner mistake