"202301203" has an extra digit 0 in between the year and month, so Excel isn't recognizing this as a possible date string for having too many characters. If it's one typo, remove that extra digit and you're done. If it's more than one cell that won't convert to a nice YYYYMMDD format, then you'll need to clean your data first and look for any values you know should be dates, but have more than eight characters.

If your sheet is laid out in a way that you can reasonably filter it and skim visually, you could insert a new column and use the LEN() function to evaluate the column's values for how long the number strings are.

Say all the dates are in column A:

1. Insert a new column B.
2. =IF(LEN(A:A)>8,"oops!","")
3. Filter on column B for "oops!" or whatever entry you want it to show you for the TRUE result, and look for a pattern of data entries that need fixing to remove that extra 0 character between the year and month in your YYYYMMDD format.
4. Fix those and then you can select-all and apply the date format.