no applicable method for 'group_by' applied to an object of class "function"

Hi @Tsuki and welcome to the community!

It's a little difficult to troubleshoot without a reproducible example of what your data look like. Take a look at FAQ: What's a reproducible example (`reprex`) and how do I create one? when you get a chance.

In addition to what @Pathmes has added, the summarise function may be getting confused where your newly created column names are the same as built-in function names.

Once you get the data read in correctly, I would try something like:

diff_in_means <- chick_data_2022_03_22 %>%
  group_by(growth_rate) %>%
  summarise(mean_growth = mean(growth_rate),
    diff_growth_rate = diff(mean_growth))