So I changed the model from stdp_synapse_hom to stdp_synapse and these are my both synapses
nest.CopyModel("stdp_synapse", "stdp_synapse_exc", {"weight":gmax,"alpha":1,"Wmax":gmax, "tau_plus": 20.0,"lambda": -0.0005, "mu_minus": 0, "mu_plus": 0})
nest.CopyModel("stdp_synapse", "stdp_synapse_inh", {"weight":-gmax/(N-1),"alpha":1,"Wmax":-gmax, "tau_plus": 20.0,"lambda": 0.005, "mu_minus": 0, "mu_plus": 0})
So now I am getting properties for lambda but still it is having some issues
NESTErrors.BadProperty: BadProperty in SLI function SetStatus_CD: Setting status of 'stdp_synapse_inh' connecting from node ID 17 to node ID 35 via port 1: Weight and Wmax must have same sign.
I tried to change signs of lambda also while setting it but still same issue