As Mike pointed out in his reply: "If the USE fails and your error handler isn't handling it properly or the user chooses to ignore, they'll get that error when the PACK fires."
I suggest also modify line
use_string = SYS(5)+CURDIR()+'\data\'+ALLTRIM(dbfarray(stepx,1))
with
use_string = SYS(5)+CURDIR()+'data\'+ALLTRIM(dbfarray(stepx,1))
Note the backslash missing from 'data\" because CURDIR() already end the returning string with a backslash so you double it... maybe this was the error that from some reasons did not "fire" and you try PACK with no table open...
HTH