If the values are actually datetimes, both of them then the interval to use is 'dtday', which tells SAS both values are datetimes and that you want the day intervals.
Anytime you get an 'error' or other message you do not understand copy the data step or procedure code along with ALL the messages generated by the step or proc, open a text box on the forum with the </> icon and paste all of the text.
Since you do not show anything explicitly using formats anywhere I suspect you may have issue with one of your variables. You may need to run proc contents on the source data set and check.
However since you used the code
data finalbicopd; set finalbicopd;
you have completely replace the Finalbicopd data set and the attributes may not be as they were.
I suspect you are getting a WARNING, not an error. There is indeed a difference. in SAS. Errors would keep the data step from running.