What signal are you using?
The AI is reporting exactly what's occurring. There's an AI driver quite aways ahead blocking the line and has to leave the block before this train can proceed. If it's in the middle of the route and not at a junction where tracks split from single to double, you should use an Advance/Permissive, or Distant signal. These would allow this AI driver to continue to drive while the AI driven train ahead moves into the next block.
The AI is reporting exactly what's occurring. There's an AI driver quite aways ahead blocking the line and has to leave the block before this train can proceed. If it's in the middle of the route and not at a junction where tracks split from single to double, you should use an Advance/Permissive, or Distant signal. These would allow this AI driver to continue to drive while the AI driven train ahead moves into the next block.