Foreach_in_collection command

You can try the following:

foreach_in_collection clk [get_clocks *] {
   set period [get_attribute -quiet $clk period]
   set_clock_uncertainty -setup [expr $period * 0.1] [get_object_name $clk]
}

I guess you wanted this:

 foreach_in_collection clk [get_clocks *] {

    set period [get_attribute -quiet $clk period]
    set current_setup_unc [get_attribute -quiet $clk setup_uncertainty]
    set current_hold_unc [get_attribute -quiet $clk hold_uncertainty]
   
    set_clock_uncertainty -setup [expr $period * 0.1 + $current_setup_unc] [get_object_name $clk]
    set_clock_uncertainty -hold [expr $period * 0.1 + $current_hold_unc] [get_object_name $clk]  
}