Using an external API

ClearAll[GermanZip2GeoPosition];
GermanZip2GeoPosition[zip_String] := Block[
   {response = Import[URLBuild[{"https://api.zippopotam.us", "DE", zip}], "RawJSON"]},
   Query["places", 1, GeoPosition[ToExpression@{#["latitude"], #["longitude"]}] &][response]
   ];
GermanZip2GeoPosition[zip_Integer] := GermanZip2GeoPosition[ToString[zip]]


GeoGraphics@GeoMarker@GermanZip2GeoPosition[90402]

Using an external API

ClearAll[GermanZip2GeoPosition];
GermanZip2GeoPosition[zip_String] := Block[
   {response = Quiet@Import[URLBuild[{"https://api.zippopotam.us", "DE", zip}], "RawJSON"]},
   If[response =!= $Failed,
    Query["places", 1, 
      GeoPosition[ToExpression@{#["latitude"], #["longitude"]}] &][response]
    , Null
    ]
   ];
GermanZip2GeoPosition[zip_Integer] := GermanZip2GeoPosition[ToString[zip]];
SetAttributes[GermanZip2GeoPosition, Listable]

Example

GeoGraphics@GeoMarker@GermanZip2GeoPosition@{90402, 90491, 90513, 90518, 90522, 90547, 90552, 90579, 90762, 90768
}

_{Code and plots done in Mathemathica 11.1.1 on Win7}