Run lightning-charge on c-lightning regtest
the problem is in your call of charged there you are passing the argument --ln-path ~/.lightning assuming that you run lightning in your home directory thus you have to set ln-path to /tmp/l1-regtest/. 

Now you say that this also did not work. So before I give you a solution let me explain what is actually going on. charged talkes to lightningd via an rpc-interface on an UNIX Domain socket. In older versions of lightningd there was a file called lightning-rpc in the folder that you specified as lightning-dir however lightningd has switched its API recently to to have subdirectories in lightning-dir for mainnet, testnet and regtest. (the reason was that in particular in the default directory people wanted to have different configs for mainnet, testnet and regtest) 

Thus I assume that you will find lightning-rpc when you do ls /tmp/l1-regtest/regtest which means that you should pass the tmp/l1-regtest/regtest in --ln-path when you call charged. If that is not the case I would have to see another error message.