The U prefix on the error code indicates this is a User error, not a system (S prefix) error. Hence you need to figure out which product / program generated the U0626 message. If you are running an IMS program, for example, you should look to see if IMS generates a U0626 and if so find the message in the appropriate manual and follow what it says to do to fix the error. The text of the message indicates a lot -- you could find in the program the I/O AREA for ARCDFILE and compare its length to the LRECL of the data set that DD name ARCDFILE points to -- that alone may help you figure out how to resolve the problem.
However, since you did not provide us much information about the problem (you didn't even tell us which language you are using), there is not much we can do to diagnose the problem.