At building custom design roller coaster in RCT1 from GOG, i cant seem to build slope track, whether it goes down or up. The option at track window forbid it (goes dark) when i choose bank turns with up and down track. How do i solve this problem? thank you

Well, duh. You need to straighten the track back before you can use regular slopes.

No, i mean the diagonal slopes, not straight ones

It's hard to say anything useful without a screenshot of the exact track construction.

Here as you can see im trying to put diagonally left downhill track, but it is blacked at the rollercoaster window. As a result an usual downhill track appears. This is no problem in RCT2, i can do the diagonal track. But in RCT 1 deluxe, i just cant.

It's just an RCT1 limitation. The only way you can go up or down while banked in RCT1 coasters is by making a helix, unless we're talking of Wooden/Steel Twister coasters.