Enter the body of text here

Code should be placed in three backticks as shown below

library(dittoSeq)
 library(scater)
 library(patchwork)
 library(cowplot)
 library(viridis)

 ## UMAP colored by cell type and expression - dittoDimPlot
 p1 <- dittoDimPlot(spe, 
                    var = "celltype", 
                    reduction.use = "UMAP_mnnCorrected", 
                    size = 0.2,
                    do.label = TRUE) +
   scale_color_manual(values = metadata(spe)$color_vectors$celltype) +
   theme(legend.title = element_blank()) +
   ggtitle("Cell types on UMAP, integrated cells")

 p2 <- dittoDimPlot(spe, 
                    var = "Ecad", 
                    assay = "exprs",
                    reduction.use = "UMAP_mnnCorrected", 
                    size = 0.2, 
                    colors = viridis(100), 
                    do.label = TRUE) +
   scale_color_viridis()

 p1 + p2
# include your problematic code here with any corresponding output 
# please also include the results of running the following in an R session 
Error in .var_OR_get_meta_or_gene(target, object, assay, slot, adjustment) : 
  celltype is not a metadata or gene nor equal in length to ncol('object')
sessionInfo( )

You did this:

p1 <- dittoDimPlot(spe, 
                    var = "celltype", 
                    reduction.use = "UMAP_mnnCorrected", 
                    size = 0.2,
                    do.label = TRUE)

And then got an error that says

Error in .var_OR_get_meta_or_gene(target, object, assay, slot, adjustment) : 
  celltype is not a metadata or gene nor equal in length to ncol('object')

Which seems pretty clear? The var argument you used is not in the colData slot, nor is it a gene name, nor is it a vector that is the same length as the number of columns of your SingleCellExperiment object. You are the only one who has that object and can check, so I would recommend you look at colData(spe) to figure it out.