What version of Photoshop?
What operating system?
You are allowed two copies, so that probably is the problem.
Best use the Cleaner tool to give PS a clean environment to install to
If Photoshop CC and you are installing via the CC Desktop Application manager that looks like this
Then right click and run as Administrator.
There are other tips etc. at
If you are using a Mac or earlier version Photoshop, please say.