Why can't I turn on Bank Transfer on my Payment Gateway page?

Bank Transfer comes from a plugin. Until it's active, its row shows Not activated and an Activate link.

1. Click Activate, or go to Plugins, find Bank Transfer and activate it.
2. Return to Settings → Store Settings → Payment Gateway and click Configure on Bank Transfer.
3. Fill in Account holder name and Account number. Both are required to enable it. Add IFSC / Routing and Bank name too, plus Branch and SWIFT / BIC if you need them.
4. Tick Enable at checkout and click Save Changes.