At first, you need to activate your vendors’ selling capabilities so that they can start selling on your marketplace. To do so, Navigate to –
WP-Admin → Dokan → Vendors
You will find the vendor list. On the right side of every name, you will find a toggle button as shown below. Simply turn on the button to make a user active or turn it off to make a user inactive. It will automatically enable or disable their selling capabilities.
Please refer the following detailed dokan documentation here – Selling Capability.