Windows Office 365 - can't install on 10.13.6

Hello together :)

I have been given an iMac from 2010 from my mom as she has a new one. On this iMac, the newest Microsoft Office 365 was running perfectly fine with my account as I can use my access on up to 5 computers. As the iMac was super slow and a lot of personal data has been collected over the years I decided to delete everything and to reinstall High Sierra. That worked perfectly fine and now I have High Sierra 10.13.6 - the highest possible version (like before resetting but now the iMac is faster). I downloaded the Microsoft Office 365 (the same as before) but it tells me that I do not fulfill the software requirement of the system and that I would need High Sierra 10.13 and higher - but I have 10.13.6?! How can I fix this problem? Why did it run before and now I can't even install it? (I can't download Word etc. from the Apple store as Word there requires 10.14., so that's no option).

Thank you very much in advance,

Best,

Jupiterlady