Q.Meyers Lemon Tree
Can I keep my Meyers Lemon tree outdoors during the winter. The lowest temperature we have had is 21 degrees. We kept it in the house during the winter and it has lost all its leaves.

This is normal when the tree does not receive enough light. They will survive this usually, though. Supplementing light at 100 or 200 watts per tree will help greatly. I am afraid that those temperatures are a little low for the tree, and you would be better off supplementing light indoors at this time.
This article will refresh you on the care of this tree: https://www.gardeningknowhow.com/edible/fruits/lemons/growing-meyer-lemons.htm