I am getting the following error during analysis. How do I fix that ?

***Error*** Modulus of Elasticity Missing For Element ...

The error means that material has not been assigned to certain elements. You can graphically find out those elements by choosing Select > By Missing Property > Missing Elasticity, as shown in the picture below

The elements with missing attributes will be highlighted. You may then assign the appropriate material to those elements using the graphical user interface.

Another quick way to fix the issue, specially of you have only one type of material for the whole model, is to go to the input command file and type in the following commands under the CONSTANTS section as shown next

For Steel:
CONSTANTS
MATERIAL STEEL ALL

For Concrete:
CONSTANTS
MATERIAL CONCRETE ALL