"BPC version is 10.1, BW version is 7.4 SP - 010 and CPMBPC is 810 SP - 0005." - Please upgrade to the latest for both!!
"Why cant I use YTD for planning? Is there any logic to use YTD or Periodic for Planning?" - The logic is simple - we are talking about P&L planning and for P&L accounts planners normally enter the results per month, not the accumulated result from the beginning of the year.
Vadim