irule syntax error

It's your quotes around "/sol". They are so-called "smart quotes". They are a character sequence that cannot be processed by the iRule loader. You can remove them, or copy-and-paste this:

when HTTP_REQUEST {
    log local0. "http uri: [HTTP::uri]"    

    if { ( [string tolower [HTTP::uri]] starts_with "/solr/spruat_slave" ) or ( [string tolower [HTTP::uri]] starts_with "/sol" ) } {
        log local0. "matched [HTTP::uri]"           
        pool mhf_pool_172.16.250.72-slave
    }           
}

Incidentally, I found these by going to the command-line and executing:

tmsh create ltm rule foo

Once I pasted in your rule, the bad characters were reasonably obvious.