I'd have to see the workbook to confirm, but I think it has something to do with your date field. It looks like you're using a month date part, but if that was the case then when you made it continuous it would show the "+" option. My guess is that Tableau can't view the time series as a legitimate continuous series. If you can share the workbook, I'd be happy to take a look.
I had no problem adding a trend line - see the attached
I'm on 2018-1 saved down to 10.2 what version are you using?
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trend line.twbx 2.4 MB