When sharing files/folders between PCs, first you need to ensure that the PCs can "see" each other (re Clintigm's post). You evidently have that already done. The second thing is whether the holder of the resource (computer B in your case) requires a password from the accessing PC (computer A). Here's how to bypass the password prompt.
On the computer on which the sharable resource resides (computer B in your case), turn OFF the password-is-required setting:
1. Go to Settings -> Network & Internet
2. Under "Advanced Network Settings", click "Network and Sharing Center"
3. On the left side of the screen, click "Change advanced sharing settings"
4. The window labeled "Change sharing options for different network profiles" is displayed. At the bottom of the menu, expand the item labeled "All Networks" (click the down-arrow)
5. Click "Turn off password protected sharing"
6. Click "Save Changes"
Note: occasionally a Windows update will reset this setting so that passwords are required again. If that happens, just redo the above procedure.
Hope this helps!