I want to use ggplot2 and reshape to draw multiple densities on the same plot, tried this is my code and error,

We do not have your data to reproduce the problem and not a lot of information to understand what you want to do. With your data (or a subset) it will be easier to help.

Could you ask this with a minimal REPRoducible EXample (reprex)? A reprex makes it much easier for others to understand your issue and figure out how to help.