If you are using space reservations, i.e. you set

dcache.enable.space-reservation=true

in your configuration file and all of your pools are in link groups, then you can only write into dCache if a link group is available for your transfer. Using the SRM you can specify the link group to write into. If you want to use another protocol like curl or xrootd you cannot specify a link group. In this case you need to use the WriteToken directory tag.