You couldn't use FORMAT because datatype filed date is nvarchar

first you must cast with datetime and you must check empty and null field dates

Select case when dates='' or dates is null then ''
else FORMAT (try_cast( dates as datetime),'yyyyMMddHHmmss') end from Da

You can insert Base data with the following statements:

drop table Da
create table Da(dates nvarchar(100)  )
insert into Da (dates) values('2023-04-23 00:00:00')
insert into Da (dates) values('2023-04-23 00:50:00')
insert into Da (dates) values('')
insert into Da (dates) values(null)

From the error message, your column is of a string-like datatype, not datetime-like. If so, you probably want to use string functions:

select 
    concat(
        substring(dat, 1, 4), 
        substring(dat, 6, 2),
        substring(dat, 9, 2),
        substring(dat, 12, 2),
        substring(dat, 15, 2),
        substring(dat, 18, 2)
    ) as new_dat
from mytable

Or using replace() multiple times:

select replace(replace(replace(dat, ' ', ''); ':' ''), '-', '') as new_dat
from mytable

The integer value returned in this conversion refers to the number of days since Jan 01, 1900. In your second query, the date you have entered is stored as a varchar, thus the conversion error you're seeing. You can cast or convert this to a datetime and the query will succeed.

Ex:

SELECT CONVERT(INT, CONVERT(DATETIME,'2013-08-05 09:23:30'))
SELECT CONVERT(INT, CAST ('2013-08-05 09:23:30' as DATETIME))