constant <clock>_period : time := 10 ns;
What is the error with this code ? it says syntax error near "<".
I think you generated a testbench-template in Vivado or ISE, as these programs generate code like this.
The idea is that you replace the
<clock> with the name of your clock-signal: If your clock-signal is called
clk, the line would become
constant clk_period : time := 10 ns;
which is perfectly fine.