If you have a donor who has only given to a fund that is considered "non-tax deductible," then a giving statement will not generate for them through the traditional route. Statements only generate for those who have tax-deductible giving.
If you want to generate a statement for those who have only paid to non-tax-deductible funds:
Go to their profile and click "giving" on the left side and then click the down arrow next to "download as excel." You will be able to produce a statement from that page. You may wish to customize the template to remove the mail-merge options for tax-deductible giving, as it may communicate $0.00 giving if you use your standard template.
If you want to generate a statement for those who have given to tax-deductible funds and include their non-tax deductible:
Suppose someone has given at least $1 to a tax-deductible fund and you would like to include their non-tax-deductible payments on a statement. In that case, this can be done through customizing the statement template to include the non-tax-deductible mail merge option.