Hi @saif_s,
It appears you are using a very old version of SILVA db parsers. The D_x__ schema has not been used in several years. You can remove that portion of the search term. We've moved to a more accurate Greengenes-like taxonomy. I'd recommend looking through the RESCRIPt tutorial on how to make your own SILVA database.
Updates on filtering by taxonomy can be seen via the tutorials. Here is one example.
Here is some history on the D_x__ schema.
-Cheers!
-Mike