I can't get pip on my python3 venv on Ubuntu 24.04

I have a fresh install of Ubuntu 24.04. I added pip with the usual sudo apt install python3-pip, and went to install some packages, but got the error: externally-managed-environment. This was all quite new to me, so I did some reading about venv. Using PyCharm, I attached a virtual environment to my Python project at /home/colin/Dropbox/codebase/Python. All seems to be going well so far. I opened up a terminal and ran:

source /home/colin/Dropbox/codebase/Python/venv/bin/activate

and now I’m in the virtual environment. Next I try to install a package with either:

pip install requests
python3 -m pip install requests

In both cases I get ModuleNotFoundError: No module named 'pip'. Okay, fair enough, my previous install of pip was for the system, and isn’t visible in the virtual environment. So I try to install pip while in the venv:

sudo apt install python3-pip

and get:

python3-pip is already the newest version (24.0+dfsg-1ubuntu1).
0 upgraded, 0 newly installed, 0 to remove and 16 not upgraded.

Huh. Okay. I’m still in the venv, and try:

which pip

and it returns:

/home/colin/Dropbox/Codebase/Python/venv/bin/pip

So what am I doing wrong here? Why can’t I install packages with pip in my venv? It might be worth adding that I was following instructions from here for all this, but it doesn’t mention this issue.