Conversion failure typically means that a value cannot be converted in a number on the receiving side. Possibly the variable source is character.
Overflow typically means that a value is too large for the receiving variable. Such as a variable that has been defined to only hold values up to 9999 but the value in question is 99999999 or similar.
You would have to examine the specific record(s), variables and the target table to determine exactly what is incompatible.