You're looking at StableBuild's Deleted PyPI mirror - where we index all deleted packages from the Python package registry.

xtts-api-server
0.5.51

This version was deleted from PyPI around 2023-12-02. Installing fails with:
$ pip3 install xtts-api-server==0.5.51 ERROR: Could not find a version that satisfies the requirement xtts-api-server==0.5.51

Install through StableBuild

You can install this package from StableBuild's PyPI snapshots from before the version was deleted:
pip3 install \ -i https://YOUR_API_KEY.pypimirror.stablebuild.com/2023-12-01/ \ xtts-api-server==0.5.51
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Or, download files manually

And install manually via pip: