I like your UIs, they are informative and neat. They also give clear instructions to the player without making them feel dumb. Just a few things I would change so they’d look better done.
Some text is too close to the edges of the background.
I would consider making the TextLabel have at least an indention sized space on both sides so it doesn’t look so cramped. Just eyeballing it would even make a difference.
The transition time between screens is a little too long.
While I like the transitioning you have of the frames, I do think that it is too long and players may begin to get bored. Additionally, speedy animations can keep players focused, making sure you can keep them in your game.
You can continue through the interface with no inputs.
Consider making the “Continue” button unresponsive if the user hasn’t provided an input or the input doesn’t meet set criteria.
You have an option to close the interface before the user is done.
I would remove this in favor of a back button so players can’t circumvent your efforts.
Also, if you’re looking for an easy way to almost perfectly position your UIs for all screen sizes, I would give this plugin a go.
Sorry if this is kind of nitpicky, just some things I think would make your system better.
The main UI when you first join seems to have an offset towards the top left of the screen, which feels unnatural at the start as there appears to be no reason for it. (Mouse is roughly at screen centre). If you want to perfectly centre something, set the Anchor Point to 0.5,0.5 and the Position to 0.5,0,0.5,0.
As A_roxn mentioned, it may be a good idea to add a gap in the circled areas, having text go directly against an edge never looks very aesthetically pleasing.
Transition time takes a while, seemed like there was also a little bit of a timed delay along with it, might want to decrease that. Adding a way to go back would be good as when I was testing it, I would have preferred a back button to view more issues. Maybe add a lower limit for the RP name to three characters or something so that people just don’t end up walking around with a single letter name tag (depends on usage of it really)
Making it so you can drag the UI would be a nice touch. Whilst the UI looks nice in colour scheme, it feels a bit bland, in all honesty. You could attempt what the person above said and add an actual blank car number plate for the background for inputting your number plate, along with changing the font type and size to replicate how it would look in the actual game, sort of it like a preview.