I'll save you some time on this one: he's almost certainly lying.
Donald Trump has offered a pointed rebuttal to anyone criticizing his lack of endorsements from GOP senators: They’re coming “very soon,” he said Sunday, “you watch.”
But:
In interviews with nearly 20 neutral senators this week, none said the businessman had even approached them for an endorsement. Most haven’t spoken to Trump in years.
Why would Donald Trump want Senate endorsements? His campaign is predicated on everyone but him being losers, and the people in Washington being exceptional losers, and glad-handing losers for their loser mojo does not seem in line with the Donald Trump "message," to the extent that such a thing exists. On the other hand, he may see these endorsements as collectable prizes, and want a few simply because he sees other people parading around with them. It's hard to say.
I can't imagine it matters. Again, the people who support Donald Trump are the people who hate governance and even their fellow Republicans so very much that they want to burn the whole thing down, or people who might not necessarily be able identify their own senators in the first place. It's important for Rubio because he needs to prove he has clout and is not quite as adrift as he seems; it's important to Ted Cruz because he needs to demonstrate that there is, somewhere, someone who has previously worked with him that does not absolutely loathe him afterward.
Trump, though? Trump doesn't need anyone but himself and a microphone.