When you create a table with a record length greater than 4000 bytes you get the error:

X139: Record size exceeds the maximum

The problem here is not the length of the field, but the size of the page which in this casemeans that you have exceeded the size of the page 4,000 bytes.

The table is obviously greater than 4000 bytes so creating a tablespace with a buffer pool which is greater will solve the problem. To do this you will need to create a tablespace for a bigger buffer:

CREATE TABLESPACE TSPC32K
BUFFERPOOL BP32K;

You can then go on to test this by creating a test table (or even your own table) by including the IN TSPC32K; in your create table statement:

CREATE TABLE TEST 
( F1  CHAR (254),
  F20  VARCHAR (6000))
IN TSPC32K;