Tidyverse: Error in as.matrix : attempt to apply non-function - r

Question

I am trying to calculate SPEI values using SPEI package and Hargreaves method. I want to automate the process so that I can calculate SPEI for all 6 stations in one go and save them to a new file spei.3.
SPEI is calculated in three steps. First, we calculate PET values (spei_pet), which is then subtracted from Precipitation value to calculate climatic water balance (spei_cwbal). The CWBAL value is then used in SPEI function from the package of the same name with a scale to calculate SPEI values.
I am new to R and very new to tidyverse, but the internet says they are easier to work on. I wrote the code below to do my task. But I am surely missing something (or maybe, many things) because the code throws an error. Please help me identify error in my code, and help me get a solution.
library(tidyverse)
library(SPEI)
file_path = "I:/Proj/Excel sheets - climate/SPI/heatmap/spei_forecast_data.xlsx"
file_forecast = openxlsx::read.xlsx(file_path)
##spei calculation
spei.scale = c(3, 6, 9, 12, 15, 24)
stations = c(1:3, 5:7)
lat = c(23.29, 23.08, 22.95, 22.62, 22.43, 22.40)
lat.fn = function(i) {
if (i <= 3)
lat.fn = lat[i]
else if (i == 5)
lat.fn = lat[4]
else if (i == 6)
lat.fn = lat[5]
else if (i == 7)
lat.fn = lat[6]
}
for ( i in stations) {
file_forecast %>%
mutate(spei_pet[i] <- hargreaves(Tmin = file_forecast$paste("tmin", i),
Tmax = file_forecast$paste("tmax", i),
Pre = file_forecast$paste("p", i),
lat = lat.fn[i])) %>%
mutate(spei_cwbal[i] <- spei_pet[[i]] - file_forecast$paste("p", i)) %>%
mutate(spei.3[i] <- spei(spei_cwbal[[i]], scale = 3))
}
It throws an error
Error in as.matrix(Tmin) : attempt to apply non-function
lat.fn[i] also throws an error, which gets rectified if I use no i. But I need to use some kind of function so that lat.fn takes different value depending on i.
Error in lat.fn[i] : object of type 'closure' is not subsettable
Thanks.
Edit: The data is in the form of a data.frame. I converted it into a tibble to give an idea of what it looks like.
> file_forecast
# A tibble: 960 x 20
Month p7 p6 p5 p3 p2 p1 tmax7 tmax6 tmax5 tmax3 tmax2 tmax1 tmin7 tmin6 tmin5 tmin3 tmin2 tmin1
<chr> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
1 Jan 0.162 0.185 0.293 0.436 0.529 0.658 26.4 26.5 26.2 25.9 25.7 24.9 9.57 9.75 10.0 10.4 9.94 9.77
2 Feb 0.207 0.305 0.250 0.260 0.240 0.186 32.2 32.2 32.1 31.9 31.8 30.9 12.4 12.7 12.7 13.0 12.2 11.9
3 Mar 0.511 0.650 0.602 0.636 0.625 0.501 37.3 37.1 37.1 37.0 36.9 36.1 18.7 19.3 18.3 18.0 17.3 16.9
4 Apr 0.976 1.12 1.05 1.12 1.17 1.16 39.5 39.2 39.6 39.5 39.5 38.8 22.8 23.2 22.5 22.2 21.7 20.8
5 May 3.86 4.12 3.76 4.29 4.15 3.84 38.2 37.9 38.3 38.1 38.2 37.6 25.1 25.4 24.9 24.7 24.5 23.8
6 Jun 7.31 8.27 7.20 8.51 9.14 8.76 38.0 37.6 38.1 38.0 38.0 37.7 27.2 27.3 26.9 26.7 26.6 26.1
7 Jul 13.9 15.6 13.2 17.0 19.1 17.8 33.9 33.6 34.0 33.9 33.8 33.5 26.8 26.9 26.6 26.5 26.4 26.0
8 Aug 15.2 17.2 14.4 18.6 20.1 18.4 32.6 32.4 32.7 32.4 32.3 32.0 26.2 26.4 26.1 25.9 25.9 25.4
9 Sep 11.4 11.9 10.5 12.9 13.2 13.1 31.9 31.9 31.8 31.5 31.5 30.9 24.4 24.6 24.3 24.3 24.3 23.7
10 Oct 5.19 5.76 4.81 5.40 5.44 5.04 29.8 30.0 29.6 29.3 29.3 28.6 20.9 21.1 20.8 20.9 20.8 20.2
# ... with 950 more rows, and 1 more variable: year <dbl>