LodgeiT

Select your client  via "Dropdown Arrow" from the left side or through  "Clients" from the right side.

Client "Settings" is located below Client's Name

To avoid future form error, make sure to complete your Client's information at -

Settings --->General Settings  

Contact Details Settings ---> Tax

Then you can proceed to add a form.

Related article:

Define Client Relationship

Adding clients Archiving Clients / Delete Client