target problem (how to solve)

Assuming that you are using DDSCAT 7.3.2:
Probably you need to increase the initial memory allocation in ddscat.par
In ddscat.par, change the line following

' Initial Memory Allocation '

to larger numbers. The three numbers refer to the max target extent in the x,y,z directions.
For example,

' Initial Memory Allocation '
30 40 50 = dimensioning allowance for target generation

allows use of targets that do not extend more than 30 lattice spacings in x, 40 in y, 50 in z.
Increase the values as needed. Note that once the target has been generated, DDSCAT will reallocate only as much memory as is required for the actual problem, so it is OK to use numbers in ddscat.par that are significantly larger than needed, as long as they don't exceed the available memory available on your system.

DDSCAT

Light Scattering Code