I agree with @chau06 . Adding a new queen to 50 bees is just a waste of money & a good young queen. Oh, & welcome to the forum.
You’ll need to start again with a fresh colony, & don’t be shy on asking questions here. It’s better to ask questions, rather than to outlay good money after bad.
You’d need at least a couple of thousand young bees, with brood, in order to succeed with a new queen.
You initially installed the flow super when you transferred the nuc into your brood box?
Delay in what? Transferring the nuc to the brood box? Or delay in transporting the nuc?
I’ve heard that it is good to give a few days to a week after assembly and painting before transferring bees into a new box (I guess VOCs/fumes/off smells?).
But I am not sure after reading your message what happened. Maybe you were supplied with a good nuc but they overheated or something else happened before you transferred them to a new flow hive with too much space for them to occupy and defend?
In any case, glad you’re continuing your learning journey. Unfortunately sometimes a few (thousand/million) bees will be sacrificed in the learning process.
Make sure you read through a good beekeeping book or two, take a class of it is available, and find a local mentor.
Welcome to the forum. Hopefully you’ll continue to find helpful advice here.
I’m a bit confused too. It sounds like there was something wrong with your nuc hive prior to installing.
When you do install your replacement nuc hive into your flow hive, make sure your hive configuration is as shown below:
The hive in the foreground is how your hive should be started - with a single brood box. The flow super is added later (middle hive in the photo) when the bee population has greatly expanded and they have drawn the comb on all the frames and filled most of them.