Solved
I have changed browsers, just to make sure it was not being blocked by security features on Firefox. No joy.
I was looking for how to enter contributions for an Individual 401(k), actually. Got this very helpful information below from TT support: Once you open your return, make sure to select Federal in the black column to the left and then select Wages and Income (or Income & Expenses if using the Self-Employed Version) at the top right. Then scroll down to "Other Business Situations" - select Show More and then select Start or Revisit for Self-Employed Retirement Plans. From there, follow the screens to be brought to the section where you can enter amounts for your Individual 401K contributions.
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