I'm having some trouble using the following function to load the gene id's from UCSC to annotate my RNAseq results - getting the following error when i try to implement it:

Error in .testForValidKeys(x, keys, keytype, fks) : 
  None of the keys entered are valid keys for 'TXNAME'. Please use the keys method to see a listing of valid arguments.

The function:

transcriptToSymbolAndDescription = function(ucsc_transcriptID)
{
    df = select(TxDb.Mmusculus.UCSC.mm10.knownGene,
                keys=ucsc_transcriptID,
                columns='GENEID',
                keytype='TXNAME')

    symbolAndDescription = select(org.Mm.eg.db, df$GENEID, c("SYMBOL", "GENENAME"))
    row.names(symbolAndDescription) = ucsc_transcriptID
    return(symbolAndDescription)
}

any help or suggestions would be greatly appreciated! :)

It is not fully clear what you exact input is, but you likely will need to use the Homo.sapiens set of annotation packages. See e.g. answer James here: Converting between UCSC id and gene symbol with bioconductor annotation resources

you can list the valid keys for the Txdb object as the ERROR suggests with keys

library(TxDb.Mmusculus.UCSC.mm10.knownGene)

txdb= TxDb.Mmusculus.UCSC.mm10.knownGene

> head(keys(txdb, keytype="TXNAME"))
[1] "ENSMUST00000193812.1" "ENSMUST00000082908.1" "ENSMUST00000192857.1"
[4] "ENSMUST00000161581.1" "ENSMUST00000192183.1" "ENSMUST00000193244.1"

If this is not the format of your given ucsc_transcriptID, than you would probably want to use a different keytype, perhaps TXID?

> keytypes(txdb)
[1] "CDSID"    "CDSNAME"  "EXONID"   "EXONNAME" "GENEID"   "TXID"     "TXNAME"  
> head(keys(txdb, keytype="TXID"))
[1] "1" "2" "3" "4" "5" "6"

The ERROR is saying none of the given ucsc_transcriptID can be found, so another quick check is

any((ucsc_transcriptID %in% keys(txdb, keytype='TXNAME')))