Hmm ok first off if you use the notation :CATEG then you need to have an actual item called CATEG. Unless there is an application item its unlikeley your item is called that way. Page items are usually called PX_CATEG. So check if the item exists under that name, and if it doesn't then use the correct name in your code.
Click Session and see if it actually has the value 'B2B'
After submitting the page, Please click on Session from 'Developer Toolbar' and check the value for the item P5_CATEG. If the value not exists in that session then your validation will not work. Plz check and confirm.
In case value is not exists for that item in session then try to create dynamic action to set the value for that item.
PS: Change your name in forum instead of number.