what does that error mean

[pimkkk]

Hi there,
i want to get the period of a structure which considered P-delta effects, so i added the following code at the end of my code.(after the load pattern) but i got an error says "missing operator at _@_ in expression...+001 -1.344782e+001....."
set xDamp 0.02; # damping ratio (0.02-0.05-typical)

set lambda [eigen 20]

set omega [expr pow($lambda,0.5)]

set Tperiod [expr 2*$PI/$omega]; # period (sec.)

puts $Tperiod

Besides, if i substitude the code above with
set mode 1;

for {set x 0} {$x<=[expr $mode-1]} {incr x} {

set w21 [eigen $mode]

set w [lindex $w21 $x]

puts "Period [expr $x+1] = [expr 2*3.1415/($w**0.5)]"
};

the error i got is "argument are not in valid range".

thank you very much