Ask the preparer of the S-Corp return.

Ask your client.

Recreate the info from looking at all years that the K-1 has been provided (that ownership has existed.

Bonus - determine if there's any 'outside' basis.

Tax year 2021 is the first year the company is an S-Corporation. It was a Partnership from tax years 2018 through 2020. The Schedule K-1 (Form 1065) provided a Basis Worksheet that was not included with the Schedule K-1 (Form 1120-S).

Then you have a starting basis for the S Corp .

Neither an S Corp or a partnership is required to provide basis schedules to the shareholders/partners although many do.

It is the responsibility of the shareholder/partner to keep track of their basis. Usually that means the tax return preparer does it.

EDIT: Remember that the shareholder won't get basis for liabilities so remove those to the extent included in the ending partnership basis schedule.

sjrcpa or abctax55

The first photo below is the amount ($69,387) the preparer of the S-Corporation return gave the shareholder for the shareholder stock basis at the beginning of the year 2021. The second photo is the2020 Schedule K-1 (Form 1065) and the third photo is the 2020 Schedule K-1 Partner's Basis Worksheet.

Do you know why the shareholder's stock basis at the beginning of the year 2021 would be $16,990 more than the partner's basis at the end of the year 2020?

No. Only the prior preparer can answer that.