The reason you are getting this that one or more rows have data that is not a valid type 131 date. How you fix it depends on what you want done. If you want to find all the rows with bad data so that you can fix them you can do
select * from @T where CAST(TRY_CONVERT(datetime, TableLocalDate, 131) AS DATE) Is NULL And TableLocalDate Is Not NULL
That will show you the rows with bad data. You can then correct the data.
If, instead, you want to have SQL ignore any rows with bad TableLocalDate values, you can do
select * from @T where CAST(TRY_CONVERT(datetime, TableLocalDate, 131) AS DATE) <=DATEADD(MONTH,@Month-1, GETDATE())
If you do that you won't get any errors and none of the rows which would have caused the error will be in your result.
Tom