Tax entry not possible in this item

What causes this issue?

You can not enter a tax amount in this item since, due to the posting
amount entered and the tax code, a total tax amount of zero has been
calculated.
This could either be due to the fact that the tax code has been defined
with a value of 0%, or because the posting amount entered (that is, the
tax base) is too small.
<ZH>Example:</>
With a tax rate of 15%, a tax amount greater than zero will only be
achieved with a base amount greater than 0.04. With values of 0.03 and
less, it is therefore not possible to enter a tax amount.

How to fix this error?

Only enter tax amounts in conjunction with tax codes for which a
percentage rate not equal to zero has been defined. Make sure that in
documents with several G/L account items you enter the total tax amount
in an item with a sufficiently high posting amount.

Error message extract from SAP system. Copyright SAP SE.