The MegaTinyCore, that I suspect you are using has several examples for setting up Timer Couter A (TCA0). When I started working with these I found it difficult to find the right register names. It is not a 1:1 translation from the datasheet, as it is with the AVR's you have programmed before.
What helped me a lot is having the header file (see attached) also open on my PC. In the header file I do a search for the Bit-name from the particular register I want to program and then I can step through the headerfile to see what is the right syntax. After a few hours you start to understand the thoughts behind it and it becomes easier.
iotn1614.h (264.1 KB)