NetBenefits Account & Personal Account

I currently have a NetBenefits 401k (and a Roth IRA) account through my employer. I have tried to open a personal Fidelity account, where I can add funds and purchase individual investments, but I keep getting the “you already have a Fidelity account” message. How do I do my own investments on my current (NetBenefits) Fidelity account? I don’t see where that feature exists on that my Fidelity account to add funds and invest - even if it’s separate from the NetBenefits investments. Is that not allowed?