Hi @Rick, I am not sure what the cause of your problem is but I found some resources which might be helpful and give some suggestions.
1) Make sure no other Tor instances are running
Since you freshly installed Tor I guess this is not the cause of your problem, but make sure no other Tor instances are running by using your “Task manager (Windows)”, “Activity Monitor (MacOS)” or the ps | grep tor command when on Linux. Terminate any Tor process before trying again.
Another more easier solution is just restarting your device. 
2) Do you have any anti-virus software?
My first guess is that you are running some kind of anti-virus or anti-malware software (like e.g Windows Defender - The default on Windows, Malwarebytes, Bitdefender, Avast, Norton, Kaspersky, Sophos, McAfee, AVG AntiVirus,…) which conflicts with Tor.
Try temporarily disabling your anti-virus software and try again.
If this works – Don’t keep your anti-virus disabled permanently but ping me so I can assist you in whitelisting Tor while your anti-virus remains protecting your system.
3) Disable Windows threat protection
Someone with the same issue asked a question on tor.stackexchange.com: https://tor.stackexchange.com/questions/20879/tor-browser-already-running-when-trying-to-start-the-tor-browser-on-windows
The answer was:
Turns out that in the Windows Security panel under Virus & threat protection Ransomware protection was enabled. This protects files, folders and memory areas from unauthorized changes. The solution is to click on ‘Manage ransomware protection’ and click Allow an app through controlled folder access.
4) See if “Start Tor Browser” Shortcut and “Browser” is Hidden
On Reddit many people with the same issue were able to solve it by deselect “hidden” for the “Start Tor Browser” Shortcut and “Browser”: https://www.reddit.com/r/TOR/comments/o9a7jw/comment/ljgsja2/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1
I have experienced this before, and Have fixed it!
go to: C:\Users%username%\Desktop\Tor BrowserAnd See if “Start Tor Browser” Shortcut and “Browser” is Hidden
Select both and left-click > Properties > Unselect ‘Hidden’ > Apply it to sub-folders aswell
Let me know if you still encounter issues after trying these suggestions.