Please try these steps one at a time.
Make sure your Safari version is compatible with Ponyhoof by clicking the top-left Safari menu and then click About Safari. Only versions 9 to 11 are supported.
Safari Technology Preview and Safari 12 are not supported due to new Apple restrictions.
Make sure your computer’s clock is correct. See https://time.is for the correct clock.
If Ponyhoof is not installed after clicking the Trust button, try using keyboard shortcuts to press the Trust button. It sounds silly, but it actually works for some.
It may be a random Safari problem and simply rebooting your Mac will fix the issue.