I have this exact same issue, including the "Plasma Fuel testing can't be done right now" message. Is this a problem with the stations's location or parts, or is this a mod (version?) issue? I can't find any answer to this reported issue.
I'm running KSP v1.12.3, with mod version v2.6.1.3, with several other mods active (using CKAN). All other experiments seem to be working correctly.