iTunes Store not opening for music purchase

If you're having problems finding the iTunes Store in the Mac Music application, note that you now access it via an item in the Music window sidebar.

To get that item to show up, you may need to go to the General tab of the Music > Settings… dialog and check off the iTunes Store box.

it's over a year since I last bought music through the iTunes Store. What MAY have happened (but I don't know as I used High Sierra to buy music) is Apple may have moved the Store exclusively to the Music app? If that's the case - it may not be - use Music on your phone or iPad to buy music then download it to your MBP.

We need much more information, such as what computer you're using, what OS, which version of iTunes, etc