board installation problem

[Devanshi15]

I am facing a problem while installing this board , I get this type of message in compilation window
The current selected board needs the core 'sandeepmistry:nRF5' that is not installed.
The current selected board needs the core 'sandeepmistry:nRF5' that is not installed.
Sketch uses 22148 bytes (8%) of program storage space. Maximum is 262144 bytes.

Though I previously installed it , My board is PCA10028 nordic nRF51422 .I want to change and configure spi pins, to customize this board further.

Please explain me how to do as I am new to coding

Thank you