The adjustment will not be able to be deleted, you will need to make any changes you need to make to it, or if you are trying to delete it you can change the amount to 0.00, then post the adjusting gift.
The error occurs because typically whenever a gift is adjusted adjust a gift it creates two line items in the unposted gift tab - one that reverses the original gift being adjusted and a second one with the adjusted gift. If only one of those is posted FIMS will not allow you to delete the other line. You will need to post the adjustment through and then readjust as necessary. When a gift is adjusted or reversed, the reversal and the adjusting gift must be posted together at the same time or deleted together in Unposted gifts, but if either the adjusting gift or the reversing gift is posted separately, the error will occur if you try to delete one of them.
To verify if the reversal was posted, check Gift History.