Hi @antonf86, there are some typos in the URL above (flter should be filter) and the quotes are being mangled by this forum, but I agree that even with the broader query (https://www.crystallography.net/tcod/optimade/v1/structures?filter=elements HAS "C"), the data block is an empty list, indicating that there are no results for this filter.
You can verify that the filter is valid by changing “tcod” to “cod”, where there definitely are results to your query: https://www.crystallography.net/cod/optimade/v1/structures?filter=elements HAS ANY "C", "Si", "Ge", "Sn", "Pb"
This might indicate that the TCOD database has a bug (paging @merkys), or is undergoing maintenance. Remember that OPTIMADE is just the format of the API, and does not make any guarantees about whether a given database should be working or have results all the time!