Hi @ZootZoot,
Welcome to the RStudio Community Forum.
Try subsetting the dataframe, to include only the ARK values, before you create the time series:
Crypto_ARK <- ts(data = Crypto.data$Close[Crypto.data$CryptoCurrency == "ARK"],
start = 2017,
frequency = 365)
plot(Crypto_ARK, main="Daily Closing ARK Stock Price")