If you’re getting ready to perform an on-premise update by importing new organizations into your existing tenant you might start to get some cryptic database errors on constraints.
The constraints themselves vary, mine was in reference to cndx_PrimaryKey_DiscountType, others I’ve seen were in relation to opportunities.
Whatever the error, you will need to install the 0.3 update to address and fix the issue so you can import the tenant.
When you install the server, this update is not applied by “Getting updates”.
The link to the patch is located here – https://support.microsoft.com/en-us/help/4490599/service-update-0-3-for-microsoft-dynamics-365-9-0 – once done, your upgrade and import will succeed.