Did President Trump post on Truth Social that if NATO allies continue to refuse help with opening the "straight" of Hormuz he would "reaffirm" his commitment to securing Greenland? No, that's not true: No such post appeared on Trump's Truth Social account, despite a viral image that appeared to show one. Google didn't index a post from Trump with the words in the viral image either.
The screenshot of the non-existent post appeared in a post on X (archived here) where it was published on March 21 with a comment that read:
BREAKING 🚨 Trump threatens to invade Greenland again if Europe doesn't help him against Iran.
This is what the image of the purported post looked like:

(Image source: "Syrian Girl" account on x.com)
This was the text of the purported post:
If our so-called NATO "allies" continue their refusal to help with opening the straight of Hormuz, which would be very easy, quick, simple and safe for them to do, now that the war has been largely won, I may have to reaffirm our commitment to securing Greenland for our national security. I sincerely hope it doesn't come to that, but we'll see. The ball is in Europe's court. We funded NATO for decades, getting nothing in return, now it's time for them to pay us back, in one way or another. I rebuilt our military, the strongest in the world, in my first term, and I will use it to its fullest extent to secure the safety of the United States, and the world, in whatever way I see fit. Thank you for your attention to this matter. -President DJT
Lead Stories searched for posts with both the words "Hormuz" and "Greenland" (archived here) on President Trump's Truth Social account and did not find any matching posts.
A Google search for those words limited to results containing truthsocial.com/@realDonaldTrump using the "site:" search operator returned no results either (archived here), indicating Google didn't recently index such a post.
All screenshots of the supposed post showed exactly the same comment, like and share counts and had the same background. If the post had been real the expectation would be that there were more screenshots circulating showing different counts and/or layouts (night vs. day mode, mobile vs. web interface...).