With Stripe Connect Onboarding, one important part of the configuration in the Stripe Dashboard is to make sure the bank account information is collected through Connect Onboarding.
If that toggle hasn't been turned on, sellers in your marketplace will not be able to add their bank account number and, unfortunately, won't be able to receive money for their payments.
What does the error look like
When adding their payment details, users in your marketplace may see the error "Bank account is not connected". It looks like this:
How to fix this situation
To fix this, you, the administrator of the marketplace, should edit your Stripe settings.
Visit this page and follow the instructions closely, in particular, the step that details how to Turn on "Allow Stripe to collect bank account information of authenticated users".
This will fix things immediately for your sellers, and they'll be able to edit their payment details to add their bank account number.