Please understand how difficult it is to help. You're asking us to diagnose a problem which is happening on your computer, but you've typed only 14 words which tell us almost nothing. We can't even see if you use Mac, Windows or Linux. To answer "could you provide some help", I can try with this message, but given so little info, the odds are not good.
But I will try to give you some help about how *you* can try to find the problem... all blind without knowing anything about your computer.
Teensy Loader has a hidden window that gives more information. Click
Help > Verbose info to access it.
After you click Verify in Arduino, it should show info like this:
01:38:03.146 (post_compile 17): Begin, version=1.46-beta7
01:38:03.146 (loader): remote connection 11 opened
01:38:03.147 (post_compile 17): Sending command: comment: Teensyduino 1.46-beta7 - LINUX64 (teensy_post_compile)
01:38:03.148 (loader): remote cmd from 11: "comment: Teensyduino 1.46-beta7 - LINUX64 (teensy_post_compile)"
01:38:03.149 (loader): remote cmd from 11: "status"
01:38:03.149 (loader): file changed
01:38:03.152 (loader): File "DS18x20_Temperature.pde.hex". 15044 bytes, 1% used
01:38:03.229 (post_compile 17): Status: 1, 1, 0, 2, 0, 0, /tmp/arduino_build_899657/, DS18x20_Temperature.pde.hex
01:38:03.229 (post_compile 17): Disconnect
On the 2nd to last line, you can see I've highlighted part of the line. This is the info about *where* on your computer Arduino created the HEX file. On Linux, it usually begins "/tmp". On Windows, it will usually be in an "AppData" or similar temporary folder. Macintosh uses a very complex randomly generated location, which is part of Apple's attempt to make Macs more secure (hackers can't easily guess where programs are creating temporary data). On both Windows and Mac, these locations may not be visible in the ordinary Finder or File Explorer, but you can use tools to access those locations. (can you understand how I'm having to write a longer message, because you didn't even tell us which type of computer you use?)
When Teensy Loader does the actual upload to your board, it will try to read the file from that folder. This error message means it could not read the file. Your job, once you know the full location and file name, is to try to figure out *why* the file is not accessible.
First, the obvious answer: if you quit Arduino, usually that temporary location will be deleted. It was created by Arduino, so it's considered to be no longer needed once Arduino is no longer running. Don't quit Arduino if you still want to access the file it created!
If Arduino is still running, the very first (and hopefully obvious) troubleshooting step is to inspect that location to see if the folder still exists and still contains the file. If the file is present, then the question becomes *why* can't Teensy Loader actually read it. But if the file has vanished, the question is what caused it to be deleted. Maybe some other software on your computer is overly aggressive about deleting temporary files which it believes are no longer needed? Or maybe something trying to improve security (like Apple's randomization of the folder names) has changed the name of the folder, but Teensy Loader is still trying to access the old location?
I don't know what's wrong. In fact, all I know is from those 14 words, which don't give much info. This explanation of how things work is meant to help *you* to solve the problem. Hopefully it helps? If you do discover what caused the problem, maybe you could take a moment to explain in a reply here, to help anyone else who has this issue in the future?