Can't Romance Sera

You can romance Sera as a female of any race, I think it's just harder as an elf (especially if you're too "elfy"), easier as a Qunari or Dwarf. Possibly harder as a mage. 
You should be able to flirt with any viable options (and in some conversations, Cassandra and Dorian if you started flirting with him and didn't tell him to stop after the quest with his dad), unless you're flagged as being in a relationship with someone.
If you want, you could send me your save file and I'll have a look at the flags in the editor, see if anything looks hinky. 
Have you done Sera's quest involving a war table mission and a trip to meet a 'friend' for a reward? Have you gone on a pranking spree with her? Had gross cookies on a roof? 

If you go through the game just doing main quests instead of going off onto optional maps and returning to Skyhold, you are likely to miss a lot of interactions with the companions (as some of them require you leaving Skyhold and returning again to activate between main quests), so that could be a reason you're not able to romance anyone, maybe? You'll also miss methods of changing their approval ratings, as there are things you can do in certain areas (as well as people to judge) which are completely missable if you just stick to the main quests.
I run around every time I get back to Skyhold (largely trying to get Cole's 'odd behaviour' things to pop up, which they seem to randomly decide to stop doing in some playthroughs) and talk to everyone, so I've never had similar issues.