UPS requires that the "Ship From Address" include a Company name field when you generate a label. In order to resolve this issue, update the "Ship From Address" in ShipStation to include this information.
Head to your Account Settings (the gear icon in the upper right).
Next, select Shipping and then Ship From Locations from the sidebar on the left.
Locate the Ship From Location you need to update, and click Edit.
On the popup that appears, enter the company information.
If you have your company's name on the "Name" field, that is fine, you can enter in any information you would like on the Company field. You could enter "Thanks for your order!" or enter a company slogan, etc. The stipulation from UPS is that this field not be left blank.