Problem
mars - ERROR - 20190507.231202 - Request validation failed: mars - ERROR - 20190507.231202 - restricted access to valid data. mars - ERROR - 20190507.231202 - For more information, look at mars - ERROR - 20190507.231202 - https://software.ecmwf.int/wiki/display/UDOC/MARS+access+restrictions#valid.
Solution
You are not authorised to access valid data.
Check the MARS access restrictions and modify your request accordingly.