This one requires 20 blocks in one chain, not one combo. Objectives are very explicit about what they mean; block, chain, and combo are all very different (chains are made of blocks, combos are made of chains).
Nearly every time I start this level, there is a 20-block chain hanging around for me to clear, and if not, it's usually close to 20. In the latter case, usually clearing one small chain can jiggle blocks around enough to, for example, bridge the gap between two large groups of one color. That said, it's uncommon for me to come across this case. I recommend taking a closer look and not bothering with clearing less-than-20-block-chains unless you're absolutely sure there's no 20 block chain there. A significant amount of work and care went into making sure that there should almost always (can't say always because the randomizer can technically spit out anything) be a suitably large chain available at the start, though. White blocks are very important for that, though, so if you clear most of your white blocks early on in failed attempts, it may be in your best interest to just restart the level.
As an example, I've attached a photo with a 21-chain that I found with the starting set of blocks. Keep in mind that many large chains (including this one) only work if you start at the right place; this chain has 3 sections to it, and you can't include all 3 sections unless you start at the top left where my cursor is.
Edit: We could always raise the number of white blocks on this stage, but I think that might make the level much less interesting in that it wouldn't make you need to really exercise your pathfinding skills like you currently have to. With the chains of 20-22 blocks that are currently in there, you need to do your best to try to get as many blocks into a chain, which can be difficult and very interesting because of the tradeoffs (which sets of blocks to include and which to exclude, and where you can start from to include the most sets of blocks in the chain). If there were a lot of chains of 25+ blocks, then there'd be such a large margin for error that these interesting problems would stop being a part of it.