The weird thing is, that I can't even choose to "do nothing"! I can only Gift it, delete it or start a war. Why don't I have the "do nothing" option?
As you can see above the mini-map in the lower right-hand corner, your unit is stacked with another unit and cannot remain in that tile. However, the Galleass cannot enter ocean tiles, but that's the only tile within one move of its current position, and it only has one movement point left this turn.
In short, you're stuck and must either delete or gift the unit or declare war on La Venta. I'm really curious what would happen if you DoW since you cannot remain on the same tile as the other unit.
This is strange. It appears that because of your lack of mobility and because you can't seem to toggle the "Do Nothing" button I would assume that (it has happened rarely) that your unit has moved on top of another unit that wasn't shown at the beginning of your turn. Only way to fix it is to wait until the other player moves their unit.