Unable to change field types when uploading new content

[PortWatchIMF]

Hi, I'm trying to upload a CSV file and I am not able to change the field types from String (assigned by default) to Double (what it should be). The reason why string is assigned by default is because the column contains a lot of NAs . This happens often with sparse data. However, it has never been an issue to make the change from String to Double when you review the fields. Now it doesn't let me me anymore as String is the only option allowed, as seen below. Any idea?

The values don't contain any string, just numbers and NaNs