This file ( 555+Triac.asc ) produced in LTspice simulator.I think your attached file is an Altium file (kindly correct me if I'm wrong) so I won't be able to open it. I am going to try downloading Altium though
You can download LTspice for free here:
https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html