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The problem is that you haven't imported the std namespace. Add the following after your includes

using std::vector; 

Or qualify the usages of vector<T> with the namespace.

std::vector<neuron> nrns; 

The problem is that vector<T> lives in the std namespace and you're attempting to use the type without any qualification or appropriate using statement. The safest fix is to explicitly qualify uses of the type with the std prefix.

std::vector<neuron> nrns; 

This can also be fixed by explicitly importing the type via a using statement.

using std::vector; 

I would avoid this approach though. Adding using statements to header files, while legal, is bad practice because it can change how items are compiled. This form safer than a blanket import of std but is still not great.

The problem is that you haven't imported the std namespace. Add the following after your includes

using std::vector; 

Or qualify the usages of vector<T> with the namespace.

std::vector<neuron> nrns; 

Or the more general

using namespace std; 

The problem is that you haven't imported the std namespace. Add the following after your includes

using std::vector; 

Or qualify the usages of vector<T> with the namespace.

std::vector<neuron> nrns; 

The problem is that you haven't imported the std namespace. Add the following after your includes

using namespace std; 

Or qualify the usages of vector<T> with the namespace.

std::vector<neuron> nrns; 

Or as others have suggesetd in the comments the more constrained using of specifically vector.

using std::vector; 

The problem is that you haven't imported the std namespace. Add the following after your includes

using std::vector; 

Or qualify the usages of vector<T> with the namespace.

std::vector<neuron> nrns; 

Or the more general

using namespace std;