Don't understand the error message

proc descript data=temp2_13 filetype=sas design=wor notsorted ;
nest county strata psu_id;
totcnt _zero_ psu _minus1_;
weight wt_n1;
var wt_n1;
SETENV DECWIDTH = 6 ;
output mean total / filename=op1_13 replace;
run;

Opened SAS data file TEMP2_13 for reading.

NOTE: SUDAAN sorted the main data set TEMP2_13 by NEST variable(s) COUNTY, STRATA, PSU_ID, observation number for
processing.

DATA ERROR
in Request 44
:
(Message 831)
There is a problem with nest variable PSU_ID=63.000000 in record 1949
It has only one _OBSNO_ whose value is 1949.000000
(Message 830)
Cannot compute variance contribution for WR design when sample size is 1

DATA ERROR
in Request 62
:
(Message 831)
There is a problem with nest variable PSU_ID=63.000000 in record 1949
It has only one _OBSNO_ whose value is 1949.000000
(Message 830)
Cannot compute variance contribution for WR design when sample size is 1

Sample size is 1 error occurred 1 times
Sample size is 1 error occurred 1 times
Note:
2 records written to file OP1_13

SUDAAN processing halted.