Perfect!! It's working!! Is jsonlite::fromJSON() (I presume you are using jsonlite?) still failing?
If this works, but fromJSON still fails, you may want to submit a bug in the jsonlite package.
In the meantime, though - once you have the text value, you can then convert a few different ways:
# will give you a "parsed" list output
httr::content(httr::GET("https://api.ibb.gov.tr/ispark/Park"), as = "parsed")
# use `fromJSON` on the string itself
jsonlite::fromJSON(httr::content(httr::GET("https://api.ibb.gov.tr/ispark/Park"), as = "text"))