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.