library(SingleR)
hpca.se <- HumanPrimaryCellAtlasData()
hpca.se
library(scRNAseq)
hESCs <- LaMannoBrainData('human-es')
hESCs <- hESCs[,1:100]

# Restrict to common genes between test and reference data:
library(scater)
common <- intersect(rownames( hESCs ), rownames( hpca.se ))
hpca.se <- hpca.se[common,]
hESCs <- hESCs[common,]
hESCs <- logNormCounts(hESCs)

Error in (function (classes, fdef, mtable)  :    unable to find an inherited method for function ‘logNormCounts’ for signature ‘"Seurat"’

Can someone help me why im getting this error?

Usually it's good form to format your post correctly with, y'know, new lines and stuff.

Moreover, Seurat is not a Bioconductor package, and none of the packages you show use Seurat. I am confident in saying that none of the code as shown above will generate a Seurat object at any point.

I will conclude by identifying the code as originating from the SingleR vignette, though you haven't copy-and-pasted it correctly or completely, e.g., rownameshpca.se).