Here's what happened :
you had a bunch of dates formatted as DD/MM/YYYY and pasted them and they were automatically parsed in the format MM/DD/YYYY leading some of them being recognized as proper dates (but not the ones you intend to have), and some of them being leaved as text.
To solve this you can copy paste the column in a text file, then import this file; when you import the data in excel, use the "text to columns" to choose the date format.
You can also change your locale date settings.
Last solution that does not involve creating a new file : copy paste your dates in a text file, add a ' in front of each line which ensures the cell content is not automatically interpreted, then parse the date from this text using the formula :
=date(right(H2,4),mid(H2,4,2),left(H2,2))
(if your text is in h2)
One last thing : some of your data in the left column is not in the correct format. You probably have a formatting problem / data quality problem in the source data you copy/pasted from.