I am unable to add payment details as a note or custom field at the bottom of sales invoice.
when select single line text or paragraph text it has option text box and when it comes to creating invoice the custom field is always blank despite what I write in paragraph box or single line when creating custom field. my older version would ask the paragraph text and what id write would come up on bottom of invoice, being payment details…
i guess you have forgot how you had setup Manager previously. 
the default themes show the payment details entered under Receipts & Payments automatically at the bottom of sales invoices. you do not have to add them manually.
if using other themes you can still add them as notes.
have you set the custom field to show on printed documents?
i would recommend you read the guides section as there have been a lot of changes to Manager. https://www.manager.io/guides/
I have checked the box to show on printed invoice.
I want it as to put my bank account details on the bottom of sales invoice.
on old version I did it as a custom field titled “payment details”, se,ected paragrapg text, and it said add text below…which was an empty white box to type in.
I do the same now and it says “description” for the white box. I type in to that and then when I create a sales invoice, the title Payment options is there, an empty white box to write in, and the text that I wrote before is under the white box… the text is under the text box as it would be on fields that’s describe under the box a sample of what to type… if that makes sense?? I want to put acc # bsb# etc etc below and been through the guides and can not find anything to help. I have found part of it and when I follow steps it doesn’t allow me to put this on invoice.
have you read the guide on setting Form Defaults?
ok worked it out, the guides could have been a bit more idiot proof but got there in the end. thank you for your help.