Thanks for writing in! Tiktok is not available in Font Awesome icons. You may need to create or upload a custom icon from Tiktok. Since you are using Pro theme, simply create a custom footer instead and replicate the footer or even design your own unique footer. You can then make use of the image element or the social icon element to upload a custom Tiktok icon for your footer.
If you are new with Pro theme, please get familiar with the custom header and footer builder by checking out this knowledge base article:
Hope this helps.