@Mariana Huerta First problem:

save

and

print

and all commands without a leading '.' must be inside a

.control ... .endc

block (see below): for BSIM4 the threshold is called

vth

. save it with:

.control
  save @m.xm1.msky130_fd_pr__nfet_01v8[vth]
.endc

Then you can print/plot with this syntax (vth is a voltage):

ngspice 53 -> print v(@m.xm1.msky130_fd_pr__nfet_01v8[vth])
v(@m.xm1.msky130_fd_pr__nfet_01v8[vth]) = 5.937983e-01

Thank you, this helped a lot!