irule syntax error

Try 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
    }           
} 

I'd comment though that the second condition will always be met if the first is true. It may be simpler to consolidate down to:

if { [string tolower [HTTP::uri]] starts_with "/sol" }