Due to the lack of supporting information, It is difficult to comment; the READ BEFORE POSTING topic is fairly clear about the need for some details in a post. A few points--
  • Try simply proceeding with the CHARMM-GUI inputs and see what happens
  • NPT with implicit solvent is what does not make sense
  • Post your input, and maybe some output; see the READ BEFORE POSTING topic for guidelines on how and when to include .txt file attachments in a post.

Rick Venable
computational chemist