Hi:
Is this your PC?
HP Pavilion p6616f Desktop PC Product Specifications | HP® Customer Support
If so, according to the specs, it originally came with W7 64 bit and 6 GB of memory.
You cannot do an in place upgrade from any 32 bit operating system to a 64 bit operating system.
What you need to do is to clean install W10 64 bit.
When doing a clean install, you will need to reinstall all of your programs and files as well, so make sure you have backed the files up and have your necessary program installation files.
Make bootable installation media using the media creation tool from the link below.
Make the 64 bit installation media.
Download Windows 10 (microsoft.com)
If you are given an option to select the version of W10 you want to install, select W10 Pro 64 bit, and enter your W10 Product key when requested.
If you are not given an option, if you are asked to enter a product key during the installation process, select the 'I don't have a product key' option, and W10 Home 64 bit will install.
Then you can upgrade from W10 Home to W10 Pro with your W10 Pro product key as follows...
Select the Start button, then select Settings > Update & Security > Activation .
Select Change product key, and then enter the 25-character Windows 10 Pro product key.
Select Next to start the upgrade to Windows 10 Pro.