Once you have a GarageBand Project open, you can't change the Tempo via "tap", that feature doesn't exist in GarageBand.
Here is a workaround A:
- Let's say you have your current GarageBand Project Open "Hit Song"
- Open a new Project (cmd+N), which will close "Hit Song"
- The Project Chooser Dialog opens, where you can use the Tap Tempo button to find your tempo to find the BPM value
- Instead of creating a new Project (you were only pretending) select the "Recent" item in the Sidebar and your "Hit Song" should be right there where you can double-click on it to re-open it.
- Enter the BPM value
Here is a slightly more expensive solution:
- Spend $199 to buy Logic Pro X
- Spend the next three month to learn the basics of Logic
- Import your GarageBand Project "Hit Song"
- Use the the built-in Tap Tempo feature in Logic
- You can even use the Touch Bar to tap the tempo (which requires to spend another $2000 on new MacBook Pro)

Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'