Cannot send a message that has been modified in outlook Microsoft 365 newly acquired!

I just bought Microsoft 365 and, cannot send a message that has been modify in Outlook and, can not synchronized my three computed on my local Network. Gmail, contacts are working on two computers but the third on, nothing works or synchronizes! Everything was working good when I had Microsoft Office Professional. I've tried many problem resolution on the Web and nothing works! I've tried Microsoft support and nothing, they are better to sell their products than to support their customers...! Can someone give me a KISS reply?(Keep It Simple Stupid) Thanks in advance. It's my first time and hoping for the best...