Firstly, there seems to be a typo; the right spelling is' Split' and not' Splt.'
Secondly, you can not directly use a Split to put it as a value.
Basically, it splits a string into pieces based on the character you defined (a comma in your case) So to use the Split string, you need to move the index to get the split string at that point.
Example:
=Split(Fields!Address.Value,",")(0).ToString()
will return the 1st value
OR
=Split(Fields!Address.Value,",")(1).ToString()
will return the second
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