Why does doing a backup on my iPhone only works on another account instead of my main account?

So recently, I have talked to a Apple Advisor with my iPhone not being about to back up via iTunes because the "Back Up Now" button via Computer is greyed out. I tried following one of the articles on the Apple Support page, but none of them worked. Because of this, I had to talk to a Apple Advisor. I tried following the Apple Advisor steps, in which he told me I had to boot my iMac into safe mode, I did that, and nothing.

I restarted my iMac and told the next step the Apple advisor told me, make a test account. Luckily, I had one, in which the account was gonna be a joke about it but I didn't really care anyways. I opened up iTunes and well, it allowed me to backup. So I did a backup, nothing went wrong, It actually successfully backup using that account. The apple advisor told me that if it couldn't be backed up, then there must be something wrong with my system files or something like that. (I didn't get a email about my case so I don't really know if that's what he said) When I went back and opened iTunes on my main account and plugged in my iPhone, It still had that "Backup Now" greyed.

Why is it doing this? Why couldn't it be backed up with my account? Is something wrong with my system files? Help? Please?