The manager is incompetent and unable to listen to other people’s opinions.
As soon as you show any ability, they go behind your back and say bad things about you to senior management.
After that, whatever proposal you put forward, they say it’s no good and that there are problems with it.
Then they privately ask to speak with you, saying that you haven’t been doing anything and that they have to pitifully clean up your mess and say good things about you to the boss, along with all sorts of other nonsense. They insult you terribly.
In public, they act as though they are very good to you, making people from other departments wonder, “How could you have such a great manager!?”
The manager directly dumps assignments they don’t want to take on onto you, waiting for you to mess up.
When it comes to vendors the manager doesn’t want to meet, they make an appointment and then find an excuse to say they are unavailable. Every so often, you end up with more work to handle, and the manager never tells you what they want to discuss with the vendors they arranged to meet. After the vendors leave, they put on an act in front of the boss, making themselves look pitiful and busy, and then turn around and “help” you solve the problem.
In short, this is a very problematic position. Unless the manager is replaced, this job is simply impossible to do.