My Feedback:
Hey SpiralGaia, this is really nice. Did you make this in Roblox Studio or did you import these into Roblox Studio? Either way, these look pretty good! Nice job. Here is some of my feedback:
1. Use of Margin and Padding
You don’t want your UI to have too much empty space or white space, but having a condensed and cramped UI layout isn’t that great or ideal either. You have the basis of good design, however, you have no margin or padding. I’d consider it good practice to give you self even padding all around the edges of your container frame.
2. Use of Font and Font Weight
You need to distinguish different parts of your UI text wise. Different text in your UI has a different priority. It’s like this: What do you want your user to see first, then second, then third, etc. This is why most applications have things like or stuff like H1, H2, H3, H4, H5, H6, etc. These priority levels in/of headings. Let’s see this effect right here on this post.
Heading 1
Heading 2
Heading 3
Heading 4
Heading 5
Heading 6
Your eye is most likely drawn to the first heading. So in your case, maybe the title of your frame or container should have priority. For example, “Inventory Full” can/should stay the same. But maybe you want to make the description text and button text another font-weight, so it looks like you using bold font right now, but maybe set it back to regular? You can also try to mix and match fonts to see what fits and works well together. 
3. Use of Opacity and Color
Right now everything feels in your face. It’s really hard for me to tell what to look at. Like for example, even your background the background design of frame/container is drawing my eye. But just I mentioned in the 2nd section of this post “2. Use of Font and Font Weight”, you want to have different levels of priority for everything. You want your users to look at something first, then something second, then something 3rd, etc. Right now, I feel like looking at all of them at once, which I can’t do. So in turn, it all just becomes distracting. So maybe lower the opacity of your background design, lower it so it’s enough to the point you can see it, notice it, and admire it. But don’t go so overboard it’s not even noticeable anymore. Besides playing around with the opacity, you can also play around with the color. For example, you can make the description text color a darker color like more of a light grey, so once again, my eyes would immediately be drawn to the frame/container heading or title first, which is probably what you want.
Alright, that’s about it. Hope this helps, and I hope you have a great day!
but the Ui is really good looking from a normal persons view (im not a professional ui designer or anything) 
for me