Cannot copy text from Mac to iPhone

Do you mean the Universal Clipboard? If so, try turning off Handoff and then back on again.

Firstly, maybe a bit obvious, but are they both on the same wifi network?

Secondly, and this is a totally baseless guess, but a few years back I had a simila suggestion and I always suspected it was because I had logged into a sandbox account for testing out subscriptions. I logged out of this and it has worked ever since. But it is possible it was fixed by an update or something else. But worth a try if you happen to have done this.

I have noticed a related issue in the last few months whereby copy and paste only works sometimes. For example, I usually use it to copy/paste a Microsoft authenticator token from phone to Mac. Sometimes it works first time, other times I have to repeat up to four times before it will see the code. It's really annoying, but it hasn't got to the point of being so annoying I've done anything about it yet. In your case, if you repeat a few times does it work or does it never come through?

Which I remembered as soon as I hit send - are the phone and Mac both logged into the same Apple accounts? I believe this is a requirement for it to work.

I wish I could help. When I first began encountering problems with this, web searches would turn up nothing definite, with vague suggestions such as restart your mac and your phone and try again--often more effort than just pasting into Notes on the mac and copying from Notes on the phone--a true head-scratcher, as the latter always works reliably and is almost certainly using the same underlying technology.

The fact that this continues to be broke in release after release tells us something, but I don't know what.

I thought they had fixed this bug but copying from one device to another would not work in the past if one or both devices had screen curtain on. Try making sure screen curtain is off on both devices and see if this helps.

There's a shortcut both for Mac, iOS, and other Apple devices, that is labeled "Copy Across Devices", it is found under the use your clipboard heading, under the gallery tab, on the shortcut app. For Mac, it will show up in the quick actions menu. Basically, it lets you share a clipboards across all of your Apple devices. This is already on your iPhone/Mac, you just have to add the shortcut so that it's active.

HTH. :-)

I copy and paste the text into a Mac email or text message to myself and receive it on my iPhone.

This bug is actually quite easy to resolve.
1. First, please ensure that your computer and phone are on the same Wi-Fi network, logged into the same iCloud account, and that Bluetooth and Handoff are enabled. This is necessary for the universal clipboard to function correctly.
2. Try restarting your phone and computer separately, which can solve the problem most of the time. But obviously, I understand that you have already tried this method.
3. The ultimate solution is to sign out of your iCloud account on both your phone and computer, and then sign back in. This should restore the functionality. But perhaps you find signing out and back into your iCloud account too troublesome. In this case, you can use other iCloud sync features or apps to observe which device's iCloud is having the issue, sign out of that device, and then sign back in. This way, you don't have to sign out of all your devices.
Ultimately, this boils down to the unreliability of Apple's iCloud, although I haven't encountered it in a long time. Hopefully, Apple will focus more on these long-standing reliability issues as it innovates. It's hard to imagine that a top company whose products are known for reliability and stability would have so many reliability issues, and that they are becoming more and more frequent.