You appear to be sending it to someone without an Apple device so it is going SMS.
I have had attachments sometimes not send via SMS (really MMS). I normally see it with pictures, so I assume it might be a size issue. Different cellular carriers have different size limits for texts, so what might work to some people may not work for all.
But, I am not really sure why it doesn't send.
A Numbers spreadsheet isn't a file. It is really a folder with files inside. That may be why it doesn't send via SMS.
Is the spreadsheet just data, without any formulas that need to be sent? You can export it to CSV format and might be able to send that.