long time whonix user and supporter here hi 
On whonix 15 i used a guide created by Desnake from Alphabay which went like this
Run the following code:
sudo apt-get install apt-transport-https -y;wget -q -O - https repo.i2pd.xyz/.help/add_repo | sudo bash -s -;sudo apt-get update;sudo apt-get install i2pd -y;sudo service i2pd start
The above command installs secure package installation with HTTPS, gets signing keys and imports them, updates packages to recognize your newly added repository, install i2pd and then start the i2pd service.
In Tor Browser or your Firefox, type about:config into the URL bar, accept the warnings and press Enter. Make the following changes:
extensions.torbutton.use_nontor_proxy true
After setting extensions.torbutton.use_nontor_proxy to true your browser might restart to apply changes. Apply the following changes where the first is the setting you have to search and after the space is the value you want to have:
network.proxy.http 127.0.0.1
network.proxy.http_port 4444
IMPORTANT: Wait 2-5 minutes depending on your network in order for I2P to connect properly otherwise if you try instantly it might say that the host is unreachable.
You are now able to browse i2p websites. On Tor Browser/Firefox it is important to add http:// infront of the i2p site otherwise you will search it instead with duckduckgo or other search engine which you have set.
For other Linux flavours find the most recent I2Pd documentation. i2pd is an established alternative i2p software which is the same as the original software however it is written in C/C++ and provides faster connection, less resource-hungry on the CPU/RAM etc. After installing i2pd either from source or from repository, you can proceed to the step where you modify Firefox/Tor Browser.
Now i can’t get it work on whonix 16 it installs correctly and when i visit say http zzz.i2p or http stats.i2p it loads them alright but when i visit other sites or alphabays url tnaefzkcnhryeusi7hdpqujqiqmnbtah3dmjcg3gvezohunjuxbq.b32.i2p it fails for some reason? I tried restarting installing reinstalling new template new connection nothing works! The same url on whonix 15 with everything installed i checked right now works perfectly? I used curl to get more information with the -o parameter and got curl: (6) Could not resolve host: If I try on the browser it says Hmm. We’re having trouble finding that site. but the site is up i can see it on whonix 15
To confirm it’s an issue with whonix i started my Tails usb and used the custom script by desnake to enable i2p on there. It worked perfectly and the eepsite was displayed while it was unreachable in whonix 16!
I’d like some help with this please!
An annoying thing i’d like to add is tor browser constantly puts https:// infront of i2p links can we add something to recognise it as .b32.i2p and not put https? Really annoying when browsing forums like zzz.i2p
