Hey @MFRW and welcome. to he square community. If you head to Payemnts & Invoices > Invoices > Settings >Invoices. THen choose your template (maybe default) and click edit in the bottom right. The option to allow ACH transfers should be nested here on the right side under Accepted Payment Methods. See Screen shot below.
If it's not there, let me know and or share a screen shot of what you are looking at.
