[Python] Why won't polars.when and then run in jupyters notebook?

im trying to insert/make a new column with these attributes in lines of then("") but i get columnNotFoundError instead for the lines of .then(). but these does not exists yet, what is wrong with the code?

df_2023 = df_2023.with_columns(
pl.when(pl.col("Tillatt totalvekt opp til og med 3500").eq("X"))
.then("Opp til og med 3500")
.when(pl.col("Tillatt totalvekt 3501-7500").eq("X"))
.then("3501-7500")
.when(pl.col("Tillatt totalvekt over 7500").eq("X"))
.then("Over 7500")
.otherwise(pl.lit(None).cast(pl.String))
.alias("Tillatt totalvekt")
)

df_2023.select("Tillat totalvekt").head()


*THE ERROR*

*ColumnNotFoundError: Opp til og med 3500*

I also tried with:
pl.when(pl.col("Tillatt totalvekt opp til og med 3500") == "X")
.then("Opp til og med 3500")

I could really use some help, read also the Polars documentation for polars.when, but it was for no help to solve this problem.

Thanks in advance!

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