“If I HAD to pick something, I'd say it's that I have trouble saying 'no' to people's requests.”

You're at a job interview and have been asked to described one of your weaknesses. You have to answer, but you want your answer to make you sound impressive. You want it to sound like this weakness is not really a problem. So you say:

If I HAD to pick something, I'd say it's that I have trouble saying "no" to people's requests.

If I HAD to (say something), I'd say (clause)

Use this phrase when someone is asking you to give an opinion about something that you don't really want to answer. In the example above, this phrase shows that the speaker doesn't really think this quality is a problem. You can use this phrase with different words that mean "say". For example:

If I had to guess, I'd say that there's probably 2 more weeks' worth of work left to do.

If I had to choose one, I'd say the Civic is probably a slightly better value for the money.

(someone) has trouble (doing something)

Another way to say this is "It's hard for (someone) to (do something). The version "have trouble ___ing" sounds a little bit more formal.