Error in as.vector(x) : no method for coercing this S4 class to a vector

You can't cbind a bunch of obscure object types.

If you want merged count tables you should do this:

mdat <- do.call(cbind,lapply(dat,assay))

Where row.names are Ensembl gene IDs and col.names are the SRR accessions.

Then run your table writing command.

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If you want the coordinates of your genes then do this to make a bed with the genomic locations:

library(rtracklayer)
export.bed(rowRanges(dat[[1]]),'gene_coords.bed')