This has been discussed previously. Don't launch the game by using the trainer OR locate the Borderlands3.exe in the MAIN game installation folder (not the currently one in sub folders) under trainer settings.
Trainer is looking in windows registry for HKLM\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full\Release to have a value greater or equal than 461808. If you are sure you have a higher .NET Framework and for some reason the trainer does not detect it you can set this registry value to 461808 or higher and the trainer will no longer prompt that message.
Let me know if you need help with that.
there is no F16 in other people keyboard too bro, in my case, i just manually tick the F16 (alt tabbing)
Item level doesn't really mean anything when it comes down to how good a weapon is or how it performs. Example would be, a sniper rifle with lower reload speed or a larger clip compared to one without those. The larger clip would raise the Item Level, but would sacrifice Handling or even Accuracy which is more desired on a sniper rifle.
Once the Gibbed save editor is released, I'm sure there will be bazzilions of combinations players will create that will not only make weapons better, but also increase item level. A trainer could probably trick the game into showing a 600 Item Level, but it wouldn't actually be 600. You would need to edit the Parts on the weapons to actually make a different item level.