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Brian (Powers) is exactly right, if the option to enable/switch to Enterprise mode is greyed out then it must be enabled on the backend of HMOL. The most common explaination for this are older HMOL accounts that were created years back when Express and Enterprise modes were two separate offerings. For a while now however, Express and Enterprise modes are both freely available with any HMOL subscription. So it is possible that your HMOL account was created in a time when this was not the case.
But I digress. Regardless of the how or why, Brian Powers is correct; all you need to do to resolve this issue is open a Support Case with our ATAC team and we can easily enable Enterprise mode for your account.