For the SSH, I do believe you need the public key on your local computer. There isnt much help on the eleven2 host support page. I believe a key component is missing.
The php errors are also due to you not being able to ssh onto your hosting provider. Once you are on your hosting provider there are php dependencies you will need to install .. pending your hosting provider allows it. You provider says it provides 'Jailed SSH'.
Any reason why you need to stay on that hosting provider?
I am currently using NetCup. (https://www.netcup.eu/)
Purchased the RS 2000 G7 SE Package which includes
Intel® Xeon® E5-2680V4
12 GB DDR4 RAM (ECC)
4 dedicated cores
60 GB SSD or 480 GB SAS
Remote console etc.
18.99€ / Month.
I am pretty much renting my own server instead of just paying for a web-host.
Keep in mind, you can, if you want to, sign up for Amazons AWS EC2 free-teir hosting. They give you a month or so of access to a virtual server running whatever OS you chose. The only thing you need is a credit card with atleast $1, in order to create the account. I have had a Amazon EC2 account for like 3 months now and still havent gotten a bill. Its worthwhile just to get the hang of installing and configuring Flarum.