"Hard routing constraints could not be satisfied"?

Note that typically the user shouldn't need to explicitly define signals as being global. The fitter will automatically choose appropriate signals to be global.

To make your c0 clock use a global signal, I think you can just use this:

set_instance_assignment -name GLOBAL_SIGNAL "GLOBAL CLOCK" -from "clocks:clocks|epll6_bb_a:a2d_clk_pll|altpll:altpl l_component|_clk0"

That'll make every fanout from c0 use the global signal. If you only want to use a global signal for the fanout to a specific destination signal, then I think you use this instead:

set_instance_assignment -name GLOBAL_SIGNAL "GLOBAL CLOCK" -from "clocks:clocks|epll6_bb_a:a2d_clk_pll|altpll:altpl l_component|_clk0" -to "DESTINATION_SIGNAL" 

where DESTINATION_SIGNAL is the hierarchical name of the destination signal.