This is a too poor developer to pay $99 per year for distribution of an open-source and free browser extension in Apple App Store. So please install it manually, GG Safari.
git clone https://github.com/sheepzh/time-tracker-4-browser.git
cd time-tracker-4-browser
Some tools are required to compile this project to an executable software for Safari.
There are several steps.
# Install dependencies
npm install
# Compile
npm run build:safari
Then there will be one folder called dist_prod_safari.
[YOUR_PATH]/Xcode.app/Contents/Developer/usr/bin/safari-web-extension-converter ./dist_prod_safari