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

Transition words would need to be worked 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"