FAQ.md

@@ -46,6 +46,8 @@
     * All grids, nodes, units etc. must be defined in the first sheets of the excel file (grid/node/unit etc.). A domain violation error indicates that in a later sheet (e.g. p_gnu_io), something was referenced, which was not defined in the first sheets. This can be either caused by a missing definition or by a spelling mistake (pay attention here: powerplant is for example not the same as power-plant). When clicking on the error message in GAMS studio, you can find the sheet that causes this error.
 * **gdxxrw failed**
     * The tool, that converts your excel sheet into GAMS data failed. That is usually caused by mistakes in the backbone command line (Did you spell the pathes right? Did you use the right amount of quotation marks?). Gdxxrw can also fail, if you have double entries in your excel sheet (e.g. twice the unit "lignite powerplant 1" in the unit sheet).
+* **symbol not in gdx**
+    * A set or parameter is not included in the input_data. Check your input data, whether the set/parameter is in the excel and has a row in the index-sheet of the excel file.
 # Pypsa-eur
 * **When using pypsa-eur, I get the error "symbolic privilege not held".**
     * You must run the shell you are using as administrator.