language is c++

Hi i need help with my code i am not sure why my start.cpp file is not working can you help explain why it was wrong and fix mistakes

i have these file ( start.cpp, start.h, activity.cpp,activity.h)

this is the error i am getting for my start.cpp file

start.cpp

start.h

activity.h

accounts.txt

start . cpp: In function 'std ::vector