business analysis Questions

What do you think are the Top 5 challenges of performing testing of APIs, I am referring only to only functional testing. I have generally come across many folks who are very good from app and business understanding but are not sure how to perform functional testing on APIs, what other reasons do you think exist as challenges to API tesing.
 

 How to use Selenium with C# in Visual Studio Code?

I am new to testing and my mentor at the company is asking me to get my base strong with regards to manual testing. I know java and made few demo projects with selenium - browser automation and rest assured - API testing . They are expecting me to have better test cases coverage and base needs to be strong . So , anybody knows few blogs/resources from which I can get my foundation strong?

I'm currently building out a Test PMO for an insurance company. We have a long term, fixed-price contract with a vendor using a waterfall model.  I'm currently working on KPIs and metrics which I will use to tell the testing story, which means both technical and business comsumers will need to find value in them. I'm conducting targeted interviews atm  but am interested in hearing if any of you have had experience in this arena.  

What kinds KPIs / metrics have you found valuable?

What kinds of chatring / mapping have you found to be confusing?

What considerations would you want to address?

Any additional comments?