Error: could not find function “unit”

This is closely related to, although not exactly identical to, arrow() in ggplot2 no longer supported , which says:

[the] grid [package] was loaded automatically by previous versions of ggplot[2] (making
grid functions visible/accessible to the user); now it’s referred to
via NAMESPACE imports instead, so you need to explicitly load grid if
you want to use grid functions (or [to] look at their help pages).

“explicitly load” here means library("grid") or require("grid") (grid is a base package, so doesn’t need to be installed separately).

unit() is a function from the grid package, so the answer above (which was about arrow()) applies.

Alternatively you can specify grid::unit(...) or grid::arrow(...) without explicitly loading the entire package.