PHP Fatal error: Call to undefined function wpseo_allow_system_file_edit()

This issue is kind of weird as it should not be able to happen.
There’s a thread on GitHub on it too and the issue reporter there found it went away when doing a fresh install, so most likely it’s an issue with the update not having completed properly, rather than an issue in the plugin itself.

I updated to 1.5.4.1 but still the same issue.