A bug exists in version 5.0 of the ShowTime theme where if a product has only one variant and you enable the display of the variant SKU on the product page, it will not show.  This has been corrected in version 5.01 of the theme, but if you're using version 5.0, here's how to fix it:

1. Click the Online Store, then Themes link on the left side of your Shopify dashboard and look for the ShowTime theme.

2. Click the the ... button and choose Edit HTML/CSS from the drop-down menu as shown below.

 

3. Open the Sections folder and locate the file product.liquid.  Click it to open it in the built-in editor.

4. Hit Control + F (CMD + F on a Mac) and search for <div class="variant-sku"></div>.

5. Replace that with <div class="variant-sku"><b>{{'products.general.product_sku' | t}}: </b> {{ product.variants[0].sku }}</div>

6. Save the file.

That should do it!