I am trying to open a simple Hello World program written in c++ in x64dbg and I get an error "Invalid PE File". The file loads fine in IDA. What might be the problem ?
Author of x64dbg here, the error you're experiencing is misleading and has nothing to do with the validity of the PE file. It simply means that CreateProcess failed for your file. This can have various reasons, once of which is the validity of the PE file. Get the latest version from here for a better error message.
Validate the process starts correctly outside of the debugger and make sure it is not blocked by for example your anti virus software.