[SalesForce] Connecting Marketing Cloud API User but says No Business Units available

If you are having multiple business units and have to connect to only one Salesforce org, you should be able to do it. In this case, the Salesforce org and data will always be connected to the primary business unit. Other business units however cannot reference Salesforce data directly.

If you enable the Multi-Org feature, each business unit can be connected to a separate Salesforce org. With the Multi-Org feature, I don't think there is a way to connect same Salesforce org to multiple business units. The connection might work in Marketing Cloud but not in Salesforce as you would face issues in fetching the tracking data in Salesforce from multiple business units.

Out of the box, you can only connect one SF instance to your marketing cloud instance. To connect to more than one SF instance you would need to have multi- org provisioned in your account. Proceed with caution as it comes with a myriad of challenges and is irreversible.

Copied in some answers i found in the success community:

some of the considerations for Multi-Org (MO) that you need to account for:

1. Each BU will need to be separately configured, even if they are connected to the same org. There is no "inheritance" of the connection concept at all in this scenario. This includes configuration of every user within BU's.

2. MO once enabled can't be disabled (just stating here for completeness)

3. All Contacts as well as All Subscribers will contain contacts from all BU's. While you can restrict access to All Subs (by the way this can be achieved via permissions as well as business rule) currently if you need to prevent access to All Contacts (via contact builder) you'll need to restrict users from accessing contact builder all together.

4. With MO each BU has it's own synchronized DE setup. If you utilize same integration user across more than 1 BU synchronized DE's will be the same for all BU's that are connected with that user (and the same org, sandbox and prod will be considered separate orgs).

5. With MO Journey Builder functions within respective BU's context, therefore tracking and contact data actually works more intuitively comparing to one org setup

6. You'll definitely need a custom preference center and would need to carefully consider all the unsubscribe/resubscribe flows, where these might be triggered from and what systems should be updated and when (e.g. maintaining CRM status vs MC subscriber status). And how to distinguish test from prod contact.

7. Your contact utilization will account for all contacts/leads/users that reside in sandbox and prod orgs. Just be mindful of that as this is one common oversight.