The package exists, but not on a channel you have — or not for your platform
Two distinct causes. Either the channel is missing (most bioinformatics tools live on bioconda, which itself needs conda-forge), or the package has no build for your architecture — this is very common on Apple Silicon, where a lot of bioconda packages are osx-64 only and simply do not exist for osx-arm64.
Add the channels in the documented order (conda-forge, then bioconda). On an Apple Silicon Mac, create the environment under emulation instead: CONDA_SUBDIR=osx-64 conda create -n myenv …, which pulls the Intel builds and runs them under Rosetta.