Why I am unable to see ‘Proceed to checkout’ button for my B2B site?

Check the Address Book Mode for the site in Sites > Settings > General Info: Checkout > Address Book Mode. If it is set to "Locations" or "Both", make sure you have at least one Location added in Sites > Locations & Departments.
 
If it is “Address Book” then check whether site user has the addresses or not. If site user did not set any address then please add it and you will be able to see ‘Proceed to checkout’ button.