Please,I am a student studying building technology in nigeria university, please can I do my master in civil/structural engineering directly after my degree?

Assuming you can find aa acceptable collage, of course.

Sir,do you mean it is possible for me?

You can do a master's in anything, but you might have to pick up undergrad classes along the way, depending. Please check with whichever universities you have considered. The admissions office (or some other office the admissions office will direct you to) will advise you as to what additional coursework you will need to take at their school or another school or online or by correspondence. You do realize that additional undergrad coursework will probably extend how many years your master's takes.