The spreadsheet was busy recalculating. Wait until its formulas finish loading, then click Add to this spreadsheet again. If it keeps happening, use Make a copy.
Add a template in Google Sheets
Add a template's tabs straight into the spreadsheet you have open with Add to this spreadsheet, or make your own copy of the template with Make a copy. Both start from the Template Library in the SheetsFinance add-on.
Before you start, install the SheetsFinance add-on and connect it to your account. See Google Sheets.
Add a template to the spreadsheet you're in
Use this to keep a template next to your own sheets, for example to point it at data you already track. The add-on rebuilds the template in your spreadsheet, so you don't need to grant access to any other file.
Open the spreadsheet and go to Extensions > SheetsFinance > Template Library.
Find the template with the search box or Filters.
Click Add to this spreadsheet on its card.
The template's tabs are added after your existing tabs and the first one opens. Its formulas start loading data straight away, using your SheetsFinance account.
Adding a template usually takes a few seconds. In a spreadsheet with many formulas it can take up to a minute, and the other Add to this spreadsheet buttons stay disabled until it finishes.
What comes across
The template arrives as it was built: values and formulas, formatting, column widths and row heights, merged cells, notes, links, dropdowns, conditional formatting, charts and images.
Charts are redrawn from the template's settings, so colours and spacing can differ slightly from the original.
Text boxes that explain the template become a note on the cell they sat over. Text boxes with copying instructions are left out, as they don't apply.
Tab names that are already used in your spreadsheet get a number, for example WACC (2), and the template's formulas point at the renamed tab. You can add the same template more than once this way.
Make your own copy
Make a copy when you want the template in a spreadsheet of its own. A few templates offer only this option, for example templates built around a pivot table.
Click Make a copy on the template's card in the Template Library, or Copy to Google Sheets on the template's page on the Templates page.
In the dialog Google Sheets opens, click Make a copy.
A new copy can show #NAME? in its SheetsFinance formulas until Google Sheets loads the add-on in that file. See Fix #NAME? errors in Excel and Google Sheets.
If adding a template doesn't work
When an add fails, the add-on removes any tabs it had started, so your spreadsheet is left as it was.
"Google Sheets took too long to add the template"
"Another template is being added to this spreadsheet"
Templates are added one at a time per spreadsheet, including from another browser tab or a collaborator. Try again when the other template has finished.
Some charts, images or rules could not be added
The template was added, and the message says what is missing. Use Make a copy to get the complete template.
The template shows only Make a copy
That template can't be rebuilt inside another spreadsheet, for example because it uses a pivot table or holds a very large amount of data. Make a copy instead.