For example if you define the element type to be C3D8 instead of DC3D8 for a heat transfer analysis you will end up with perhaps eight errors on the .dat file. Often the problems are created by editing the input file and not copying and pasting the correct value.
For example if you want to apply a load to PickedSet12 in one of the steps be careful to make sure if is _PickedSet12 or just PickedSet12 by looking into the Assembly section and noting the number of underscores that precede the set name. If there are two underscores in the Assembly section then you need one underscore in the Step section. Similarly one underscore in Assembly means no underscore in Step. It is important to make sure that what you reference in each step is the same thing that the program reads from the Assembly information.
Another common problem can be not defining hourglass stiffness. Solve that problem by deleting an R at the end of the element type. For example C3D8R becomes C3D8.