Is there any prayaschitta for not carrying out an Ekadashi fast in the scriptures?
There is actually one. Because of Bhima's giant appetite he was not able to fast for a complete day, so Krishna explained that if could fast once a year on Jyestha Shukla Ekadashi, which is the hottest and longest solar day of the year, it would be equal to have fasted all the various ekadasis of the year. So, this is considered a sort of prayaschitta for breaking an ekadasi vow during the year, many considered this to remove the faults from previous year.
This account is given in the Brahma-Vaivarta Purana