Hi, when i run simulations using DSR in 802.15.4 i got this error:

[wpan/p802_15_4phy.cc::PD_DATA_request][20.615044](node 99) Invalid PSDU/MPDU length: type = DSR, src = 99, dst = -1, uid = 3, mac_uid = 1660, size = 159

I know that it is because the packet max length is 127, but im using 50 so i dont know how to fix the problem.

Try Setting the energy Parameters. In my case below parameters worked...
set val(energymodel_15) EnergyModel ;
set val(initialenergy_15) 3.0 ;# Initial energy in Joules

set val(idlepower_15) 0.45 ;#LEAP (802.11g)
set val(rxpower_15) 0.9 ;#LEAP (802.11g)
set val(txpower_15) 0.5 ;#LEAP (802.11g)
set val(sleeppower_15) 0.05 ;#LEAP (802.11g)

$ns node-config -adhocRouting $val(rp) \
-llType $val(ll) \
-macType $val(mac) \
-ifqType $val(ifq) \
-ifqLen $val(ifqlen) \
-antType $val(ant) \
-propType $val(prop) \
-phyType $val(netif) \
-channel [new $val(chan)] \
-topoInstance $topo \
-energyModel $val(energymodel_15) \
-idlePower $val(idlepower_15) \
-rxPower $val(rxpower_15) \
-txPower $val(txpower_15) \
-sleepPower $val(sleeppower_15) \
-initialEnergy $val(initialenergy_15)\
-agentTrace ON \
-routerTrace OFF \
-macTrace ON \
-movementTrace OFF

Good suggestion; except the question was posted EIGHT YEARS AGO.