Does the error message take you back to this screen?
If so, remove the entry or entries by using the Delete button rather than removing the entry by using the Backspace key.
Then select Federal Review down the left side of the screen to see whether the error has cleared.
If the error clears, then re-enter the property tax paid being careful not to enter any leading blanks or '0's.
If the error does not clear, post back here and post more information.