When you have a character and you can't come up with a story for them.

I go the other way around.

I come up with a story idea, plot it all out, do page by page breakdowns and have place holders for characters.

Then I create the characters to fit into the requirement of the story and develop the characters along the story as it progresses along the preset path.

I actually come up with characters and the story together. I flush both out as I come up with ideas on how both sides will work together. Characters are super important to the story for me, and if you have trouble coming up with a story make a list of what the character likes, and maybe that will spark a story.
The current story I'm working on started out with me just drawing random panels to get quick pages out but I grew a story from there.
The main character likes to eat a lot, so I developed a plot around her eating something that she shouldn't have and what happens in the aftermath of that plot point. I grew the world and story from just this idea alone because I decided the main character loved to eat food and sweets.
Not sure if this helps but I will gladly clarify anything!

It can be quite difficult to reverse-engineer a plot to serve a character as characters are usually meant to serve the plot but here's some ways you might be able to have a go at it:

1) Think about your character's personality and positive/negative qualities (if you have written those) and try to create a story which challenges some of their negative qualities and perhaps jeopardises their positive ones. This allows for character development.

2) Write a whole plot first, with placeholders for your other characters too, then work your character in as a key person within the plot. You could have the plot be entirely irrelevant to your character then change your character to fit it.

3) If you feel you are completely unable to work your character into any sort of plot, don't be afraid to "kill your darling" and rework him until he fits, you may be attached to him now but trust me he'll become so much more to you when you know he has a purpose of some sort. You can always recycle aspects of him your really like and turn him into an entirely new character altogether, but I understand a lot of people are reluctant to do this sort of thing, which is totally cool too!

I hope those three options help, there's no right or wrong way to write a story but there are definitely methods that make matters easier for you and allow you to reach your full potential as a writer. Best of luck!

I think you should start by defining the world the character lives in. Start defining their relationships, their home, their enemies, their moral compass. Give them a sandbox to play in and the story will come. Try not to force it, or else it will never feel satisfying.
Character driven stories need the character to be able to bounce off of their world and other characters, they need aspirations and thought patterns. Just play with that and you’ll find your story.
Good luck!

Put them in a drawer and wait for the right script.

I usually take the "character first" approach - work out the character's personality and background, etc. and then figure out a story for them to work in. Like with my character Steamroller Man, for example - he's a character who wants to do good and be a hero, but his power is basically destroying things, so he often does more harm than good. Since my comic is a comedy, I started trying to think of situations that would be funny to put that character into. From there, just letting your mind wander a bit and daydreaming about that character/setting combo will suggest other details like other characters to interact with, etc. Before you know it, you've got the bones of a story!