Note about LOG_ARCHIVE_DEST:
For Enterprise Edition users, this parameter has been deprecated in favor of the LOG_ARCHIVE_DEST_n parameters. If Oracle Enterprise Edition is not installed or it is installed, but you have not specified any LOG_ARCHIVE_DEST_n parameters, this parameter is valid.
You cannot use both LOG_ARCHIVE_DEST_n and LOG_ARCHIVE_DEST, as they are not compatible.

Try :
alter system set log_archive_dest_1='location=C:\oracle\oradata\mydb\archive MANDATORY'