Sprout SEO 3 with Craft Tags
For any Elements that do not have URLs, like Tags or Users, you'll need to tell Sprout SEO what metadata to use.

Also, be sure that the do craft.sproutSeo.meta tag comes before the {% sproutseo 'optimize' %} tag. The do tag prepares the metadata for the optimize tag to process:

{% do craft.sproutSeo.meta({ 
  section: 'Self Help Tips',
  title: tag.title,
  description: metadata.metaDescription|striptags|slice(0, 150),
  canonical: craft.request.url,
  ogTitle: tag.title,
  ogType: 'article',
  ogUrl: craft.request.url,
  ogImage: metaImage.url(),
  ogImageWidth: metaImage.width,
  ogImageHeight: metaImage.height,
  ogImageType: metaImage.mimeType,
  ogSiteName: siteName,
  ogDescription: metadata.description,
  twitterCard: 'summary',
  twitterSite: '@twitteruser',
  twitterCreator: '@twitteruser',
  twitterTitle: tag.title,
  twitterDescription: meta.search.description,
  twitterUrl: craft.request.url,
  twitterImage: metaImage.url()
}) %}

{% sproutseo 'optimize' %}


We do have on our list to simplify this scenario and allow you to add an Element Metadata field to the field layout and just pass the elementId.

{% do craft.sproutSeo.meta({ 
      elementId: 123
}) %}

{% sproutseo 'optimize' %}