Hi Maru,
You are right, the in-place eDiscovery & hold feature is available in the Office 365 Enterprise E3.
According to the error message, the root causes of the issue may be related to the following factors:
1. Exchange Online (Plan 2) license is not assigned. Please re-confirm the option is selected:
2. The user has no corresponding permission. Please make sure the problematic account has been added to the Discovery Management role group.
1). Go to your Exchange admin center.
2). Select permissions on the left panel.
3). Double-click Discovery Management role group.
4). In the Role Group window, click the plus icon under Members section to add your account.
5). Click save.
After following these steps, please sign out, and wait for several minutes, then sign into Exchange admin center again to see if the same issue will occur.
Thanks,
Vivian Wang