How to interpolate missing values given a SpatRaster object in R Terra package?

Question

I would like to interpolate the missing values (set to 0) within my SpatRaster object.

I created a SpatRaster object and assigned values to specific cells depending on cell coordinates:

raster_layer <- rast(ncol=22, nrow=11, xmin=-3819.721, xmax=1132.362, 
  ymin=-698.7546, ymax=2013.745)

for (i in 1:nrow(Test)) { #Test is the sample dataset I am pulling the coordinates 
and the cell values from.
  col_num <- Test$grid_column[i]
  row_num <- Test$grid_row[i]
  cell_value <- Test$pm1_mgm3[i] 
  
  raster_layer[row_num, col_num] <- cell_value
}

Plotting the raster layer gets me this result and I want to interpolate all missing values:

Raster layer

I initially used a solution from another similar question from this question:

r_interp <- raster_layer %>%
    as.matrix() %>%
    apply(MARGIN = 2, \(x) approx(x)[["y"]]) %>%
    raster()

plot(r_interp)

However, I keep getting errors like "[rast,list] none of the elements of x are a SpatRaster" or "Error in h(simpleError(msg, call)): error in evaluating the argument 'x' in selecting a method for function 'rast': argument "FUN" is missing, with no default".

Reading the Terra documentation doesn't seem to help clarify how to write a simple interpolation function for a SpatRaster object.