mac 2007 wont install el capitan AND doesnt accept username and password

It is very probable that the bootable USB stick that you bought is an older version whose certificate has expired.

The error message "no packages are available to install" is an indication of that. In October 2019 the certificates

for several mac OS's expired so they will not install.

Apple released an updated version of El Capitan with a valid certificate which can be downloaded from here,

How to upgrade to OS X El Capitan – Apple Support

Apple never released El Capitan on a USB, it was and still is only available as a download.

However there is a workaround that might get your El Capitan on your mac using the USB installer you bought.

We are going to set the time and date on your mac to a date previous to the expiry date of the certificates.

Connect your mac to your router via cable, not WiFi

Boot to the USB installer, click on Utilities in the menubar select Terminal.

Make sure WiFi is switched off, it can reset the date back to today.

Enter a new date, for example or just copy and paste

sudo date -u 011421002015

press Return

enter your password

press Return

If Terminal returns an error saying sudo : command not found, then try again without sudo.

Go to Install OS X and press Continue, the installation procedure should start. Follow the prompts.

If this works then when the OS is installed and booted up you can Open System Preferences> Date & Time

and reset the time back to today.

If this does not work then it may also be due to the fact that the mac may need an earlier version of Mac OS X

installed before it can install El Capitan.

You may need to install Mac OS X Snow Leopard (10.6) if you are searching make sure you get a genuine

Apple Snow Leopard Retail copy. This will probably install version 10.6.3, after installing you need to be

updated to version 10.6.8 which you can download from here,

https://support.apple.com/kb/DL1399?viewlocale=en_US&locale=en_US

Once you have downloaded that and installed it open the App Store and check for any outstanding updates,

install them.

Having done that then you will be able to download the version of El Capitan that has the new certificate,

click on the link further up.

Here are the instructions for downloading and installing that newer version of El Capitan.

Go to Section 4 and click on Download OS X El Capitan.

This downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running El Capitan,

a mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install El Capitan

but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which 

you will find in your Applications folder, it should be 6.2GBs in size.

 (If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install El Capitan on, that is for later.)

To start the installation of El Capitan double click on the Install OS X El Capitan.app.

The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the InstallMacOSX.dmg

if you need it in future, or you could just make a copy of the Install OS X El Capitan.app prior to installing and moving it to

an external drive for safe keeping.

It is also possible to create a bootable USB installer disk using the Install OS X El Capitan.app

in the Applications folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

Is this an iMac, MacBook, MacBook Pro, eMac? A 2007 iMac can install and run .El Capitan. However, I don't know if it can be installed without first installing Lion. However, you need to provide more specific information about the failure such as exact error messages. Also, from where did you get the El Capitan installer? I the USB stick a bootable installer or do you just have a copy of the actual installer on a USB stick? If the latter, then you cannot create a bootable installer without a working Mac.

Upgrading to El Capitan

To upgrade to El Capitan you must have Snow Leopard 10.6.8 or Lion installed. Download El Capitan from the App Store. Sign in using your Apple ID. El Capitan is free. The file is quite large, over 5 GBs, so allow some time to download. It would be preferable to use Ethernet because it is nearly four times faster than wireless.

      Macs that can be upgraded to OS X El Capitan

         1. iMac (Mid 2007 or newer) - Model Identifier 7,1 or later

         2. MacBook (Late 2008 Aluminum, or Early 2009 or newer) - Model Identifier 5,1 or later

         3. MacBook Pro (Mid/Late 2007 or newer) - Model Identifier 3,1 or later

         4. MacBook Air (Late 2008 or newer) - Model Identifier 2,1 or later

         5. Mac mini (Early 2009 or newer) - Model Identifier 3,1 or later

         6. Mac Pro (Early 2008 or newer) - Model Identifier 3,1 or later

         7. Xserve (Early 2009) - Model Identifier 3,1 or later

To find the model identifier open System Profiler in the Utilities' folder. It's displayed in the panel on the right. Are my applications compatible? See App Compatibility Table - RoaringApps.

No 2007 model can run El Capitan. You haven't provided information on the exact model your have. It should come with two Software Restore DVDs with the OS version you can install. Without those disks you may not be able to install any OS obtainable from Apple other than Lion which works on some, but not all, models from 2007.