You will only be able to select the company that is associated with your primary client. Make sure you have the company created and the primary client on that matter is a contact (employee) of that company.2. In the Overview page, under Details tab, search and select the company you want to assign this matter to.
Alternatively, if the company that should be the primary client on this matter has no contacts associated with it, you can link company to a matter and mark it as a primary client. The matter will then automatically be converted into a company matter.