Unable to open installation page

I’m trying to download wordpress onto a clients website in a secondary folder so that it does not affect their current website until a switch over takes place. The fils have all been installed, extracted and edited where necessary however when I go online to complete the installation it says that it cannot find the page.
When I did this with another client I deleted all the files and started again, this worked perfectly however I’ve tried it on this and it just does not want to cooperate!

Any ideas?!

Thanks!