u1db-serve: 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/bin/u1db-serve - from package: u1db-tools

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

Error in other languages:
u1db-serve: bevel nie gevind nie
u1db-serve: comandă negăsită
u1db-serve: comando não encontrado
u1db-serve: commande introuvable
u1db-serve: command not found
u1db-serve: käsku ei ole
u1db-serve: không tìm thấy lệnh
u1db-serve: komanda nerasta
u1db-serve: Kommando nicht gefunden.
u1db-serve: kommandot finns inte
u1db-serve: komut yok
u1db-serve: nie znaleziono polecenia
u1db-serve: níor aimsíodh an t-ordú
u1db-serve: no se encontró la orden
u1db-serve: no s'ha trobat l'ordre
u1db-serve: opdracht niet gevonden
u1db-serve: parancs nem található
u1db-serve: perintah tidak ditemukan
u1db-serve: príkaz nenájdený
u1db-serve: příkaz nenalezen
u1db-serve: команда не найдена
u1db-serve: командата не е открита
u1db-serve: 命令找不到
u1db-serve: コマンドが見つかりません