First of all, make sure that the extension and specifically the Product Attachments tab are enabled in the module's general configurations (Stores → configuration → Amasty Extensions → Product Attachments). In case you've chosen to include attachments in orders, we would advise to also enable the Display 'Include In Order' Files on Product Pages and Widgets setting:
As a second step, it’s important to select the customer groups that will see the attachments on the storefront:
Please, keep in mind that the attachments tab is not displayed when a product doesn’t have any attachments. Hence, kindly check that you’ve assigned files to a product or a category:
Finally, it’s worth noting that Magento perceives attachments as downloadable products. Therefore, we also recommend that navigating to Stores → Configuration → Catalog → Catalog → Downloadable Product Options and setting Use Content-Disposition to Inline: