It seems that sqlite3 (standard command line tool to work with SQLite databases), is nowhere to be found in the default search path of your QNAP.
However, I found it to be install several times ...
With SSH, go to your QNAP and cd to the root and do a search for sqlite3:
cd /
find -name sqlite3
Depending on the storage capacity on your QNAP this may take a while, and possibly produce a list, something like this:
[/] # find -name sqlite3
./share/CACHEDEV1_DATA/.qpkg/CloudLink/python/lib/python2.7/sqlite3
./share/CACHEDEV1_DATA/.qpkg/CodexPack/usr/lib/python3.6/sqlite3
./share/CACHEDEV1_DATA/.qpkg/Qmono/bin/sqlite3
Many many more were listed - caution hower: some of these are directories and not executable, so verify that before linking!
You could make a link in the directory you're working in (just an example - you may not have mono installed):
ln -s /share/CACHEDEV1_DATA/.qpkg/Qmono/bin/sqlite3 .
Or even better, in a location that is found in (or added to) the PATH environment variable.
I did notice that "/opt/bin" seems common, but I did not have that directory so I had to create it and it to the env variable.
mkdir /opt/bin
export PATH=/opt/bin:$PATH
cd /opt/bin
ln -s /share/CACHEDEV1_DATA/.qpkg/Qmono/bin/sqlite3 .
You'll have "sqlite3" now available no matter where you're at 😁