Hi,
it's because Ansys is organized in "modules", or "ambients":
"BEGIN" is the first "gateway" to Ansys. If you use Unigraphics, then you may be familiar with the "Gateway" concept. It's the "ambient" where you setup the generic configuration options. You are in the "BEGIN" whenever you start a new job or whenever you finish (NOT exit !!!) operating with another module (via the command FINISH).
"PREP7" is the preprocessor. Please note that even in Workbench, if you wanted to insert a Commands snippet you might need to re-enter the preprocessor and it would be called exactly the same. There are a lot of commands which are PREP7-specific, they do NOT work with ANY other module. In APDL, you call the preprocessor with the command "/PREP7".
"SOLU" is the solution module (the solver ambient). Once again, there are commands which work only within SOLU.
"POST1" is the "single-step post-processor", the one where you can access ALL nodal and element results of any kind, for ALL the nodes at the same time, but ONE substep (timestep) at a time. Some commands are valid only in POST1, of course, AND THIS IS THE CASE OF THE PRNSOL, PLNSOL, ETC COMMANDS !!!
"POST16" is the "time-history post-processor", the one where, when applicable, you can retrieve results for SOME selected nodes only, but over the whole timesteps in your analysis.
There are also some minor modules, such as the "Auxiliary" ones which serve principally for 3D-solid models import, etc...
Of course, there are lots of commands which work indifferently in all the modules.
The structure of ANSYS should be very well described in the Help, please have a thorough reading of the Manual before attempting "semi-advanced" operations such as the ones you are trying...
However, the solution (!) to the error you get is simply to enter the post-processor POST1, either by GUI pick, or by adding the /POST1 command in the APDL before issueing the PLNSOL command. Please also note that you must be sure to have the results file loaded in memory. This is automatic after a SOLVE command, otherwise you have to load the results file AND read-in the loadstep (SET) you need.
Regards