joking aside, Trump is allowed to tell his diplomats not to testify, because this is not an actual (aka. legal) inquiry, which has to be voted on by the House initiated by the House judiciary committee, to start an inquiry. The fact that the House Intelligence committee, which is allowed to have closed meetings not open to media, is handling this "inquiry" does cast doubt on the potential impeachment inquiry. When the House does conduct a House vote to start the inquiry to make it official, then Trump would not be able to disallowed subpoenaed witnesses.
OK, back to creating silly memes!!!