Can I still trigger conquer and hold effects even when you can't score yet on forbidden waste?

Assuming you mean Forgotten Monument ("Players can't score here until their third turn") — No. While that restriction is active you cannot Conquer or Hold there, so no conquer/hold effects trigger (FAQ #4900, CR 469–471).

The key distinction: "can't score" blocks the entire scoring action (Conquering and Holding are both forms of scoring), not just the point gain. Cards like Tianna Crownguard that say "can't score points" still let the score process happen and trigger effects — but Forgotten Monument is the stricter kind, so no triggers.

One note: Forbidding Waste is a different card — it only applies a −2 Might penalty to a unit defending alone, and has no scoring restriction at all, so conquer/hold effects there trigger normally.