Hey SaltSeaShark,
It sounds as if you are trying to connect to your iMac for file sharing over your local network and attempting to log in using the user account name and password for that iMac but it is not allowing you to connect. Is that right?
One thing to check here would be to see if the iMac is set up for file sharing, since it is not enabled by default. Use the steps in this article - OS X El Capitan: Share your files with other Mac users
Note that you can set up file sharing for all users of the iMac or select specific users. Be sure that you have set up the correct user(s) to share.
Also, in the System Preferences > Sharing panel > File Sharing select the Options button. In the next window, you should at the least have Share files and folders using AFP selected. (Windows users would normally connect using SMB.)
If you want to limit which protocols can be used to connect to your computer using File Sharing, click the Advanced Options button in the Sharing pane. You can then select which protocols are used by your Mac. By default, OS X Mavericks and later automatically enable SMB and AFP for compatibility with Windows computers, Macs using Mavericks and Yosemite, and Macs using older versions of OS X.
Once having made sure this is set up correctly, use the steps in this article to connect to the iMac's shared volume - OS X El Capitan: Connect to shared computers and file servers on a network
Thanks for using Apple Support Communities.
Happy computing.