In one of the universities I am interested in, I found a research center specializes in studies related to Africa. My research is about Africa and I am not looking for funding because I already have financial support. I only need an acceptance. Can these centers offer any help since my interest is in line with theirs?

Yes. If you have your own funding, I would highly recommend you talking with potential supervisors before you apply, in order to decide if you want to work them. The potential supervisor can then help you through the process.

Thank you
To be honest, I have no specific supervisor in mind. I was rejected many times to the extent that I feel like I need someone to facilitate this process for me.

Look at recently published work, see what matches with your own interests and contact the authors from that University. Have a short (A4 page) outlining your own research interests and the types of methods you would like to use, highlight similarities with their work i.e. 'I see you looked at ethnographic data and this is similar to how I would approach XYZ' and see if you can get a face to face (or telephone in these times) meeting. You can also see if they have any talks coming up, with conferences being virtual it could be a good opportunity to attend a talk

Good luck

Is the problem that your previous qualifications don't meet the entry criteria? Have you had any feedback on why you are being rejected? A research centre is unlikely to be able to get the university to bend the rules if there's a real problem with your application.