It is extremely common for MMB to have options greyed out for things that can actually be done. MMB has limited functionality and you often just have to call.

For example, when I book a paid CW ticket, the option "upgrade with miles" is usually greyed out, even though it is possible to do this.

Another recent example for me is an F redemption where the "change date/time of travel" option was greyed out. I'm fairly sure this was because of a manual intervention that had occurred by phone, because I'd upgraded this from a CW redemption (without wanting to cancel the ticket and start again). Similarly, when I booked an MFU by phone (it was one that ba.com couldn't cope with) a lot of options and info in MMB weren't right.