usearch: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/lib/qiime/bin/usearch - from package: qiime

You may install the required package with command apt-get install {package}

Error in other languages:
usearch: bevel nie gevind nie
usearch: comandă negăsită
usearch: comando não encontrado
usearch: commande introuvable
usearch: command not found
usearch: käsku ei ole
usearch: không tìm thấy lệnh
usearch: komanda nerasta
usearch: Kommando nicht gefunden.
usearch: kommandot finns inte
usearch: komut yok
usearch: nie znaleziono polecenia
usearch: níor aimsíodh an t-ordú
usearch: no se encontró la orden
usearch: no s'ha trobat l'ordre
usearch: opdracht niet gevonden
usearch: parancs nem található
usearch: perintah tidak ditemukan
usearch: príkaz nenájdený
usearch: příkaz nenalezen
usearch: команда не найдена
usearch: командата не е открита
usearch: 命令找不到
usearch: コマンドが見つかりません