If a texture is missing, your render will still complete, but with a pink placeholder texture instead. To fix this - make sure all textures are uploaded with the steps below:

Upload a .blend

If you have small textures/external files

  1. Save your project
  2. Select File > External Data > Automatically Pack Into .blend
  3. Save the project again
  4. Upload the .blend file

Upload a .zip

If you have large textures/external files, like videos

  1. Save your project in a new folder
  2. Copy the textures/external files to that folder
  3. Select File > External Data > Make All Paths Relative
  4. Import the files into your project
  5. Save the project again
  6. ZIP the whole folder, then upload the .zip file

More information can be found in the Documentation.