If you trying to truncate a table with the command below:

db2 alter table <table_name> activate not logged initially with empty table

and you got the message below:

DB21034E  The command was processed as an SQL statement because it was not a
valid Command Line Processor command.  During SQL processing it returned:
SQL1596N  WITH EMPTY TABLE cannot be specified for "tableX". 
SQLSTATE=42928

it is, probably, because this table has children,so another option to truncate this table is with the following command:

db2 load from <null_file> of del replace into tableX nonrecoverable

where <null_file> is, example on Linux: /dev/null

After that you have to verify what are the tables which are in pending integrity:

db2 "select substr(rtrim(tabschema)||'.'||rtrim(tabname),1,40) from syscat.tables where status = 'C'"

If you have any table in set integrity, apply the command below:

db2 set integrity for tableX immediate checked

You will probably see the message below:

SQL3601W  The statement caused one or more tables to automatically be placed
in the Set Integrity Pending state.  SQLSTATE=01586

Apply the command above again:

db2 "select substr(rtrim(tabschema)||'.'||rtrim(tabname),1,40) from syscat.tables where status = 'C'"

db2 set integrity for tableX immediate checked

When you will not see more results to the select below:

db2 "select substr(rtrim(tabschema)||'.'||rtrim(tabname),1,40) from syscat.tables where status = 'C'"

DONE!!! you truncated your table successfully.