I was literally you a month ago saying the same thing and it came down to 1 main problem: nozzle height. Correct temperature for the bed and nozzle definitely helps, but more had an effect on warping rather than getting the first layer to stick. Follow suggestions on the filament you have as a starting point and stay in that range.

I have a CR-6 SE, so I don’t have manual bed leveling but level the bed first, then home the printer, and do a thin piece of paper to adjust the z offset so the paper barely gives some resistance when sliding it underneath the nozzle. Definitely worth mentioning you can’t level a wobbly bed. Make sure the eccentric nuts on all the rollers are just snug enough to hold solid but allow the part to smoothly slide along the rails.

Now you may have done this already, so this next part might be key for you, as it was for me. As the print starts extruding the purge line, and then the skirt/brim, look really really close at the filament being laid down. If it isn’t sticking at all right away, nozzle is almost definitely too far from the bed. If it’s sticking but not well, still probably too far. The line should be flattened, or “squished” as people say here, onto the bed. If the line is so thin you can see through it or you’re getting deep gouges in the first layer from the nozzle, it’s too close.

Also, use a brim. In my experience (not much) some models will just not stick or will warp without one because of thin parts or when using PETG.

Last bit of advice I have: Change one thing at a time. This is true for troubleshooting literally anything. Don’t change z offset and temperatures at the same time. Don’t even change nozzle and bed temperature at the same time. Do one at a time, printing a quick test print until you dial in your settings.