Please revise the sentence below.

You mustn’t take all those green, blue and red dotted lines too seriously. Only thing I can see wrong is, it’s alliterative

…...as in “is, it’s alliterative"

I think that the MS grammar checker feels that 'who' is an inappropriate relative pronoun describing 'lists'. It isn't of course. There is nothing wrong with the sentence, Tangy.

If you are a native speaker of English, go with your intuitive feel for the language.

I am tempted to agree (in fact, I do agree) but I could raise an objection.

If it's a list, then there is (are?) more than one "available".

So you could make the verb plural. But that might sound a little clunky.

As part of its programming MS Word highlights common grammatical issues such as usage of who/whom, etc. In this case, it doesn't mean necessarily that it's wrong but it points it out and relies on you to review it.

My take at this is that the more natural sounding choice would be the better one - 'is', rather than 'are'. The implication is that on that list it tells the status as to which individual (singular) is available.

As to the usage of who or whom, I might chose 'whom'. However, I'm right about 51% of the time about who or whom. Be aware that most English speakers and writers get this usage wrong 90% of the time.

The reason native speakers get it "wrong", Ragman, is because the rule is wrong. 'whom' is a relic of an old case system. You don't say that we get all those wrong when we use the modern forms of English's new system.