You can download the latest version 2.4 of Petri Net Toolbox from its official site Petri Net Toolbox for MATLAB (tuiasi.ro).
NOTE : As mentioned on the home page of Petri Net Toolbox website, the current version of the Petri Net toolbox is compatible upto MATLAB 2020b.
If you face any installation related problems, please refer Petri net toolbox installation problem - MATLAB Answers - MATLAB Central (mathworks.com).