It really depends on the error you receive. It is possible that an Anti-virus program, Or Firewall that you have activated is block PerX as it is viewed by some as a threat.
If you have an anti-virus program or such running, Please add it to your exceptions list.
If not; Try another injector, A variety can be found here:
CrossFire Spammers, Injectors and Multi Tools - MPGH - MultiPlayer Game Hacking
There are no actual requirements as far as I am aware of.
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If the injection process isn't working, Please follow this tutorial ;
https://www.mpgh.net/forum/287-crossf...ing-hacks.html
If your hack still does not work, Then it is likely detected.
Good Luck!