The error message is likely referring to the SMD file mentioned above the error message in the compile log:
"pastkiller/anims/pastkiller_idle.smd"

Look on line 9 in that SMD file.

https://developer.valvesoftware.com/wiki/$model_%28QC%29

This line explains that you are missing a file:
ERROR: c:\users\briana\desktop\ponies_source\qc\rainbowdash.qc(3): - could not load file 'F:\Nikon tiedostot\projects\source\ponies/bodies/reference_RD.smd'

• DARK MODE - Changeable color theme.
  
• Option to save Settings to Data (or user-given) folder beside Crowbar.exe (like in previous versions), allowing Crowbar and settings to be placed on portable drive. Also, allows non-admin users to use Crowbar while having it installed in an admin folder such as "Program Files".
  
• Check for Data folder (like in previous versions) for Settings file.
  
• Windows file and folder context menu actions as requested here.

• Update an item where you are a contributor (for games that allow this).
  
• Edit images and videos.
  
• Add and remove contributors.
  
• Edit Required DLC, Required Items, and Links
  
• Drop-down for selecting language.

• Have way to use vpk to package model files.
  - Model mdl file name field auto-set to compiled model mdl file name
  OR
  "Select compiled model" button
  - Fields:
  - Release folder
  - Author
  - Steam account
  - Steam group
  - "Pack" button
  - "Pack" creates addon folders and files, calls vpk.exe
  
• Option for "Write multi-file VPK".

• "Compare with 2 Viewers" that opens HLMV twice with them set to show side by side.

• Modify values directly in an MDL file:
  - internal model path file name (one or two places)
  - ani path file name
  - cdmaterials paths
  - includemodel path file names
  - bodygroup names
  - hitbox bodygroup
  - various flags, such as ambientboost and mostlyopaque
  - GoldSource: $flags and $texrendermode values
  
• Scale several VTF files.
  
• Convert between VTF versions.

• Options for studiomdl compiler found in various versions of studiomdl.exe
  
• Before running compiler, check for various syntax and other errors, and report as needed.
  
• Compile from BMP or other image formats to VTF.
  * StudioCompiler's VTF Export tab and VTFEdit features are related to this.

• Options for editing a QC file.
  * StudioCompiler's Model Compile tab > Compile Model button is related to this.
  
• Options for editing materials/textures.
  * StudioCompiler's Material Compiler tab is related to this.
  
• Rotate a mesh and skeleton by a user-specified angle (to deal with Maya using y-axis as up).
  
• Rotate a bone by a user-specified angle in all frames of all animations.
  
• Edit a bone in all frames of an anim.
  
• Scale an anim or all anims.
  
• GUI generator instead of text editing of QC and QCI files.
  
• Flip viewmodel weapons (between left-handed and right-handed). Change ref mesh and anims.