CHOOSECOLS gives Value error with Structured References

Maybe a dumb question but I’ve only recently begun experimenting with smart tables. Right now I am trying to perform a CHOOSECOLS on a table. It works when I just hard-code the column numbers into the formula, but if I try using structured references I get the Value error.

So basically:

= CHOOSECOLS( Table1, 3, 20)

Works fine.

= CHOOSECOLS(Table1, Table1[CategoryD], Table1[CategoryP])

Gives Value Error. I also tried:

= CHOOSECOLS(Table1, Table1[[#All],CategoryD]], Table1[[#All],[CategoryP]])

Also gives value error.

Can anyone explain my error?