I took one Lexapro and feel nauseated with diarrhea. Can I stop taking it?

Yes, you can. These are very common side effects when starting an antidepressant/anxiolytic and they generally resolve within just days to a week or so. You can ask your pharmacist to recommend an OTC remedy to help control them.
As Jimmy said, you could also ask your doctor for a lower dose to take temporarily as your system adjusts to the medication.
If you need to be taking this medication you'll want to see if this initial period will resolve quickly for you so that you can enjoy the benefits.
Best regards and I hope you're feeling better day by day.

Yes, you can after just one dose. Here is what I did to avoid any bothersome side effects at the start of treatment. I started on just 2.5 mgs for 7 days. Then, I bumped it up to 5 mgs. I am still at 5 mgs and slowly starting to realize some benefit. I may or may not need to go higher. SSRIs take a while to work, and they can come with some annoying side effects, but these effects usually wane with time on the drug. HTH and good luck!