Open the asset for editing.
Check if the texture is actually not there. It could be something simple as the texture not being in the proper format. The texture needs to be referenced as the .texture and not the actual .tga file. This is referenced in the config.txt file and it's a simple change by changing the name in the config.txt file from shingles20.bmp to shingles20.texture.
This is another issue too with bmp files.
Ensure that the bmp is in 24-bit format and not anything else and not compressed.
Without seeing the actual error, it is difficult to say exactly what the fix is. It would be helpful if you could copy and paste the error here in the forums along with the KUID for us to investigate. To post an error message, highlight the text in the error, copy it by pressing CTRL+C (or go to the Edit menu and choose Copy), and then paste the error here in the forums.