12/27 i use stream install SCP:secret laboratory but 12/29 i turn on my notebook i find System64 and System and mono folder in my Notebook D Drive , I have uninstalled SCP:secret laboratory ,should i delete this three folder?
Yes, it should be safe to delete those folders. They are a requirement for SCP:SL.
If you're worried about breaking something, I would recommend moving the folders to see if something breaks (unlikely), and once you're sure everything is still OK, you can safely delete them.