Can I use a function in a concurrent code?

Why didn't you just try using it and let the compiler decide?

You can use a function pretty much anywhere, eg., I use them for setting (calculating) generics like address widths based on the number of registers, etc.,and that code occurs right at the beginning of the architecture.

If you're having trouble using functions somewhere in your VHDL, post example code. If you're violating some feature of VHDL, I'll look in the standard and see what it says about your case.

Cheers,

Dave