.pkg can’t be installed because its digital signature is invalid
please help me, I'm trying to install webwork app on my macbook pro running OSX 10.11.6 El Capitan and I’m getting an error “.pkg can’t be installed because its digital signature is invalid”. I really need this app for my work, please help and advice. I already tried some steps from the internet and it didn’t work for me T_T
MacBook Pro 15″, OS X 10.11