If a teacher writes that the paper lacks transitions, the writer needs to work on?

Depending on what you are writing

If it is a story, you are jumping around too much in it and no logical ordering to it, so you would need to find a way to make it work together like in chronologically or another form.

Or if it is a research paper you do not tell when you change topics, try using words like "After All" "Next" "For this reason" "on the other hand"

Transition words would need to be worked on