Why AWLEN signal is optional for master in AXI4 protocol ? If it is optional for master then how slave will get that signal because it is required for slave ?
AWLEN is the AW channel "length" signal, indicating how many transfers will be in this transaction. It has nothing to do with an address width enable.
AWLEN is described as "optional" only if the manager design is going to use the "Default" value of all zeroes, so a single transfer.
See table A9.3 in the AXI spec, where it states...
"The following tables show the AXI required and optional signals, and the default signal values that apply when an optional signal is not implemented".
So where an optional signal is not implemented, the subordinate design can have that input tied to the default value.