picture link doesn't work.

If it is a gap between the thigh halves, I can suggest a couple of fixes:

1.  If you can take the thigh apart again, do so and drill out the snap fit sockets.  Use glue to rejoin the parts after test fitting to make sure the halves mate properly.

2.  If disassembly is not possible, take a piece of sprue that the thigh was from, heat stretch a thin filament, trim, then glue into the gap using a hot glue like Tamiya Extra Thin.  Let dry, then carefully sand the sprue flush to the part.  Start with 400 grit and go to progressively finer wet sandpaper and keep going until about 1000 grit.  Then hit all parts with a clear coat, which should make things look uniform.  Some friends who don’t paint the major areas of their Gundam models do this, and it works well.  It also works to hide sanded out sprue gate points.

HTH

For me, the fix would be to use Green Stuff putty (officially known as Kneadatite).  

What is nice about this product is you have a good two hours to work with - 24 hours to fully cure.   With just your fingertip and some water, you can easily contour it with the surrounding parts, such that you won't have to do much, if any, sanding afterwards.  That's a bonus, no worries about losing surface details.

regards,

Jack

alternate

Well as a new builder I mess up on alot of things and one mistake that I made that really bothered me of creating this small gap between this character's thighs, this has been bothering me for quite some time, I originally thought of putting paint over it but I would need to obvious buy paint for that will it work? or anybody has a solution to this problem?  

Sorry, your image link doesn't work, so I can't see your figure. 

I'd have to ask some questions first: 

1. How big/what scale is this figure?
2. What material is it made from-metal, resin, or styrene?

My solution would depend on those things.

For example, for metal or resin figures, I'd likely use a 2-part epoxy putty.  Aves Apoxie Sculpt is the brand I use, but there are others.

For a styrene figure, depending on the size of the figure and the size of the gap, I'd use either bits of styrene, like sprue or pieces of styrene sheeting (eg Evergreen), stick them in the gap, then flow liquid styrene cement over the gap.  Or I'd use a putty, like Squadron white, or Perfect Plastic Putty, to fill the gap.  Once the filler has cured, then it can be sanded or filed to match the surrounding contours.

Regarding the Squadron putty, it has a lot of detractors these days, but I have never had a problem with it.

As far as covering the gap with paint is concerned, that probably will not work.  That is, the gap or seam will likely still be visible, after you paint it.

Hope that helps!

Best regards,

Brad

Baron,

If the subject kit is a Frame Arms Girl, they are molded in color.  So the OP probably wants to fill a gap in such a way as to avoid having to resort to painting, which would then not match the surrounding area:

https://www.1999.co.jp/eng/image/10660895

I normally don't wade into the anime girl pool, but am interested in this one: 

https://www.1999.co.jp/eng/image/10642143

Don't know if this what you are after , but l have seen a lot of guys on utube, ,use sprue goo to fill in the gaps .

alternate has probably been busy, but it would really help once he visits here again and posts a picture.

I had to chuckle, ruefully, when I saw the subject of this thread, "Problem with gap on thigh". I definitely have that problem. On March 5, 1966, a bullet fired by a Viet Cong or North Vietnamese infantryman did a superb job of removing a large chunk of my inner right thigh, including muscles, blood vessels, nerves, and skin. I definitely need some filler! I wonder if the maker could supply replacement parts. 

Boy have you guiys been "had".