I am not a FlexTool expert. However, there might be a clue in your error message... C;/Users/Administrator/AppData/Roaming/SketchUp/SketchUp 2018/SketchUp/Plugins/f3d_flextools.rb;22;in `register_extension' Is your normal Windows user-name 'Administrator' ? Usually you'd choose something like 'Wenzhi' ? Assuming it is your normal Windows user-name... When you initially installed SketchUp did you do it in the proper way ? Even if you [as a normal Windows user] have admin-powers you must install complex apps this way... [Note that if SketchUp is already installed you should close it, then choose the Repair option offered...] Find the SketchUp installer exe file - probably in your downloads folder ? Select that exe file's icon. Right-click > context-menu > "Run as administrator" When prompted choose "Repair" and agree to all prompts thereafter... [If it's a fresh install choose "Install" !] Installing it in any other way - even if you have full admin-powers is not the same thing. Double-clicking on the exe file's icon might 'Run' it, but NOT as 'the administrator'. It's not recommended to run SketchUp while logged in as the full 'system-administrator' - but if you have another user-name called [confusingly] 'Administrator', probably with admin-powers that IS OK, BUT you will still need to use "Run as administrator"... as explained... If you have really messed up permissions on the C:/Users/Administrator/AppData/Roaming/SketchUp path then you might also need to fix then using Right-click > context-menu > Properties > Security > Edit and ensure that every entry has 'Full Control' set and that this permissions trickles down to apply to all subfolders and files in the tree...