Yesterday, I was do clean install Ubuntu Mate 18.04.Everything is installed and Software Center with Software Boutikue,but now when I go System>Administration there is only Software Boutikue ...Software Center NO!???How to fix that i traed to reinstal Software Center,and nothig the same thing again 
Too I failed add Unetbootin,Openshot and Screenlets ppa-s,strange all this worked in Ubuntu Mate 16.04.4...please help thanks in advance 
Or how add old Ubuntu Software Center,that was vorked in 16.04 
Software Boutique is not useful