Edit: Scott beat me to it.

Does the following get you closer?  You can highlight and run just the SELECT part to ensure that the data looks correct prior...

Unfortunately, without knowing the error(s) you are getting it is hard to provide any meaningful advice.  Also, I don't have an environment where I could attempt trying to recreate the...

Here is a start at a rewrite but with the errors in your code I am not sure what else you may be needing.

WITH Base AS...

Does this do what you expect?

DECLARE @AlterSQL NVARCHAR(MAX) = 'ALTER TABLE ~tablename~ ALTER COLUMN ~columnname~ NVARCHAR(~maxlength~) ~nullstate~;'
  , @SQLCmd  NVARCHAR(MAX)
  , @SQLParm  NVARCHAR(MAX)
  , @TableName ...

And what is the end goal of what you are trying to accomplish?  Is that really that difficult a question to answer?