With rare exceptions, Mathematica/Wolfram Language input is not valid in WolframAlpha. WL is a structured programming language, while WA accepts freeform natural-language input. I'll cover how to get the answer you want using both.
Since WolframAlpha parses natural language, you don't need to provide heavily-structured input. You can get the coordinates for ZIP code 20121 using the input 20121 coordinates. Note that had you just used 20121, WA would have interpreted it as an integer, but with the added "coordinates" specification, it knows you're providing a location.
The input you originally provided is code for the Wolfram Language, a structured, symbolic programming language. You can evaluate it for free using the Wolfram Development Platform (previously Wolfram Programming Cloud.) Unfortunately, neither WL nor WA can usually provide geolocation accuracy down to a specific address, so the next best thing is coordinates for the geographic center of the ZIP code:
FindGeoLocation["20121"]
Output: GeoPosition[{38.8247, -77.4449}]
Both the Wolfram Language and WolframAlpha are excellent, just for different things. I encourage you to explore further! Here are some resources:
