itunes wont let me purchase music again

In the Music app menu bar, choose Music > Settings or Preferences > General and insure Show iTunes Store is selected.

Then click in the search box at the top of the left sidebar and then click the iTunes Store option at the top right of the main window as shown in the example in Buy music from the iTunes Store - Apple Support

If the 3 options don't appear at the top right of the main window, 2 things to try:

1) If the View menu in the menu bar contains "Hide Playing Next" or "Hide Lyrics" then click that item so that it changes to "Show Playing Next" or "Show Lyrics."

2) If there is a very small < at the top left of the main window (to the right of the sidebar search box), click it one or more times until there is no < there.