I have a list of gene symbols for which I want to retrieve the Ensembl Gene ID. I have a dataframe (df_biomrt_tmp) containing a column with the symbols and after running

df_biomrt_tmp-> getBM(attributes = c("hgnc_symbol", "ensembl_gene_id"), 
    filters = "hgnc_symbol", 
    values = df_biomrt_tmp$hgnc_symbol, mart = ensembl)

it throws the following error: Error in getBM(attributes = c("hgnc_symbol", "ensembl_gene_id"), filters = "hgnc_symbol", : target of assignment expands to non-language object

The question is: why? I have run the same test with a number of Ensembl Gene IDs to retrieve HGNC symbols and that works flawless. I suspect it is BioMart that is acting up, but any clarification is more than welcome.

I think you have the assignment operator (<-) the wrong way round. It should be df_biomrt_tmp <- getBM(...

I can't reproduce.

> library(biomaRt)
Warning message:
package 'biomaRt' was built under R version 4.3.2 
> library(org.Hs.eg.db)
Loading required package: AnnotationDbi
Loading required package: stats4
Loading required package: BiocGenerics

> symb <- keys(org.Hs.eg.db, "SYMBOL")[sample(1:10000, 75)]
> symb
 [1] "COPG1"     "GSTM1"    
 [3] "MTHFS"     "IFITM3"   
 [5] "ESPL1"     "KCNE1"    
 [7] "KARS1"     "ZSCAN12"  
 [9] "CD24P1"    "S100A3"   
[11] "PRB3"      "MPL"      
[13] "CDK1"      "ASNS"     
[15] "TMPRSS11D" "IFI27"    
[17] "MMP7"      "PPP3CA"   
[19] "ARHGAP44"  "KCNC3"    
[21] "AP1S2"     "KCNA4"    
[23] "GATD3"     "NPM1P19"  
[25] "ATP2B2"    "BUD31"    
[27] "TRNI"      "RAB10"    
[29] "EXOC5"     "CYP3A7"   
[31] "ELF2"      "MBL2"     
[33] "CST7"      "CLDN14"   
[35] "MMP1"      "AOAH"     
[37] "CCNG1"     "CACNA1F"  
[39] "ZNF443"    "CTNNA1"   
[41] "ECH1"      "TRGVA"    
[43] "BTF3"      "FAM30A"   
[45] "OR1E1"     "CLNS1A"   
[47] "TTF1"      "FKBP1B"   
[49] "STBD1"     "IK"       
[51] "KRT18P1"   "CRIP1"    
[53] "NELL2"     "RPS6KB1"  
[55] "CASP8"     "TOMM34"   
[57] "PIM1"      "CBX4"     
[59] "PRSS8"     "CRISP2"   
[61] "RASA2"     "ZNF227"   
[63] "POLR3A"    "GOLGA3"   
[65] "RPL41P2"   "H4C8"     
[67] "HGS"       "NELFB"    
[69] "FZD3"      "NOS2"     
[71] "DCTN1"     "PTCH1"    
[73] "SGTA"      "SLC7A11"  
[75] "RPS4X"    
> mart <- useEnsembl("ensembl","hsapiens_gene_ensembl")

> z <- getBM(c("hgnc_symbol","ensembl_gene_id"), "hgnc_symbol", symb, mart)

You might consider providing the symbols you used. Or maybe just try again, which is often all it takes.