Can't get Win10 to open even with new password.

I got a replacement P70 from Lenovo and put my SSD drive from my original machine in it thinking I could just boot it up and run the system with all accouterments the old one had had. No dice. It would not boot because I had gotten used to using a PIN number as a short-cut for password, and I forgot the password. The new system didn't know anything about PIN number but wanted the password which I no longer had. Then various tech support places told me to get a new Microsoft password and that would work; so I went through the hoops and got a new Microsoft password which on testing worked on a number of things but would not turn on my Win 10 at bootup. Since then after many, many frustrating tries I may have changed my Micorsoft password five or ten times. To no avail. All my files are backed up; so I am ready to reinstall a clean Win 10 on the new machine and then recover my files. I have tried to reinstall Win 10 using the media creation tool, but I get to the part where it says "where do you want to install windows?" Select the partition that you want and select the formatting option you want to perform" and then it shows me two drives: one (I think the SSD Drive) with four partitions and the other (an HDD Drive) with 5 partitions. I pick partition x because it says Windows, and then it says "can't install on partition x the reason being "Windows can't be installed on GPT partition style". How do I find out which partition to put it in and what formatting style? and how about GPT? Also I fear that eventually I will be asked for the Microsoft code numbers for my installation which I do not have. The original came to me from Lenovo with Win 8.1 installed which I converted to Win 10. Any constructive ideas appreciated. Thank you,.