Whenever I need to go to the bank, the parking lot is always full, but when I don't have any errands to run, there're always lots of parking spots available.
この場合、次のような言い方ができますよ。
ーWhenever I need to go to the bank, the parking lot is always full, but when I don't have any errands to run, there're always lots of parking spots available.
「銀行に行かないといけない時はいつも駐車場は満車なのに、用事がないときはいつも空いている」
to have errands to run で「用事がある」
ご参考まで!