data.table in R : the columns to join by must be specified using 'on=' argument error

Question

I new in R language. i have two data.table like these :

rts:
          id   rt
   1:   10_1 3180
   2:  10_10 1680
   3: 10_100  720
   4:  10_11 1060
   5:  10_1