Hi Slefevre,
It's by design, you have installed Small Business Edition.
Small Business contains only three tabs under Admin page, I have attached SBE 12.1 RU1 MP1 screenshot for reference
As per your comment "I did upgrade for SEPM 11.0 about 6 months ago but everything appears to running fine since the upgrade"
--> It's not possible to upgrade from Enterprise Edition (11.0 ) to Small Business Edition, it's a downgrade.
I guess you might have upgraded from SBE 12.0 to SEB 12.1 (12.1.671.4971), however in that case also you won't see Server tab.