There is a manuals link at the top of the page. If you click on it, pull up the CICS Application Programming Reference manual, and look up the READ command you will find the following LENGERR possibilities:
Neither the LENGTH option nor the SET option is specified on a READ command for a file with variable-length records or for a BDAM file with variable-length or undefined-format records.
The length of a record read with the INTO option specified exceeds the value specified in the LENGTH option; the record is truncated, and the data area supplied in the LENGTH option is set to the actual length of the record.
An incorrect length is specified for a file with fixed-length records.