Other reasons why PACK might not work: 1) The table is read/only, 2) There is not enough disk space.

To avoid the VFP error, and perhaps put your own error, you can use ON ERROR:

local lcOnError, llError
lcOnError = on("error") && save previous ON ERROR
IF USED("mytable") AND ISEXCL("mytable")
   SELECT mytable
   on error llError = .T.
   PACK     <-------- this part is giving error
   on error &lcOnError  && restore previous ON ERROR
   if llError
      MessageBox("Unknown error during PACK.")
   endif
   USE IN mytable
ENDIF

You can also use aerror() to get more specific information about the last error.