There are direct flights to Marrakech from many of the largest cities in Europe. Our recommendation is to look for flights with a connection via a European city such as Paris, Amsterdam or London.
Round trip flights from Seattle as an example that time of the year can be found for around $1,000-$1,300.
You may be able to find lower rates when planning a stay in Europe and booking separate flights from the US to Europe and from Europe to Morocco giving you the opportunity to spend a day or two in a city in Europe.
Alternatively, there are direct flights to Casablanca in Morocco from New York, Washington DC, Miami and Montreal. You may also want to consider this route and book a flight with a stop in Casablanca or land in Casablanca to catch the 3-4h train to Marrakech.