I believe there are two steps that may have been overlooked in your setup:
1) From your screenshot, Mike’s mjs513/RA8875 (RA8875_t4 branch) doesnt appears to be in your Arduino/libraries folder. Please download the zip from his repository, and unzip it into your Arduino libraries folder. Download from here:
https://github.com/mjs513/RA8875/tree/RA8875_t4
2) From the pastebin error log, it appears that the original RA8875 library still exists in your “hardware” folder. This may conflict with Mike’s library so please cut and paste that folder to somewhere else like your desktop: C:\Program Files (x86)\Arduino\hardware\teensy\avr\libraries\RA8875