Can't open itunes

I have an iMac desktop (late2009) for some reason iTunes will no longer open. when ever I try to open the log into apple ID screen pops up. I enter my user name and password (which works on my ipad and the website) but when I hit sing in the feild just goes blank and wont let me log in. iTunes wont open unless I log in. I thought I might need an update but I am running on Yosimity and it says I need the new OS Catalina but apparently my computer is too old. It works just fine except Itunes and I am not buying a new comuter just for itunes. any recommendations on what I can do?

Thanks