Posting a screenshot showing the problem would be useful.
I can see no reason why you shouldn't be able to buy a DOFF with your token, unless you were looking at the wrong section (anything other than the diplomacy section). I managed to buy a tier 1 diplo DOFF yesterday just fine.
There is a bug which we have a fix in the pipeline for where it is trying to slot it directly into your active roster. This may be causing the issue; try removing someone from the ground or space active roster as appropriate.