Tara on "Add on my site"

First, try:
- deactivating ALL plugins temporarily to see if this resolves the problem (plugin functions can interfere). If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
- If that does not resolve the issue, try switching to the unedited default Twenty Fifteen Theme for a moment using the WP dashboard to rule out any theme-specific issue (theme functions can interfere like plugins).