Vim 8.1.2250 does not ship with filetype detection for solidity. (One can confirm with :edit $VIMRUNTIME/filetype.vim and then :global /sol/.)
ALE has Solidity linters, but they require the filetype to be set.
The simplest solution is to create ~/.vim/ftdetect/solidity.vim and add
au BufNewFile,BufRead *.sol setfiletype solidity
However, this plugin ...
The fact that you are getting this message on your verbose log file is expected.
The Vim source code has an explicit comment about that:
* When using ":silent! cmd" ignore error messages.
* But do write it to the redirection file.
I linked to the code in Vim 7.4, but Vim 8.2 still has the same exact snippet and exhibits the same behavior.