I tried with the flags too but still got the same error.
First 10 line of my sam file is as follows:
@HD VN:1.0 SO:unsorted
@SQ SN:chr1 LN:248956422
@SQ SN:chr2 LN:242193529
@SQ SN:chr3 LN:198295559
@SQ SN:chr4 LN:190214555
@SQ SN:chr5 LN:181538259
@SQ SN:chr6 LN:170805979
@SQ SN:chr7 LN:159345973
@SQ SN:chr8 LN:145138636
@SQ SN:chr9 LN:138394717