Trying to add a footnote (just copy & pasted from the sample notebook)
{% raw %}For example, here is a footnote {% fn 1 %}.
And another {% fn 2 %}
{{ 'This is the footnote.' | fndetail: 1 }}
{{ 'This is the other footnote. You can even have a [link](www.github.com)!' | fndetail: 2 }}{% endraw %}
… and getting this error:
Liquid Exception: Liquid syntax error (line 886): Unknown tag 'fn'
SOLVED:
Just had to update fastpages!
I had an error that wasn’t quite the same as this, but it got me to this page.
For me the issue was that Jekyll didn’t like some Latex equation that I had written so that is another thing somebody could look into if they are seeing Liquid syntax errors 
jekyll_1 | jekyll 3.9.0 | Error: Liquid syntax error (line 851): Variable '{{length}' was not properly terminated with regexp: /\}\}/
jekyll_1 | Traceback (most recent call last):
jekyll_1 | 48: from /usr/local/bin/bundle:23:in `<main>'
jekyll_1 | 47: from /usr/local/bin/bundle:23:in `load'
jekyll_1 | 46: from /usr/gem/gems/bundler-2.3.9/exe/bundle:36:in `<top (required)>'
jekyll_1 | 45: from /usr/gem/gems/bundler-2.3.9/lib/bundler/friendly_errors.rb:103:in `with_friendly_errors'
jekyll_1 | 44: from /usr/gem/gems/bundler-2.3.9/exe/bundle:48:in `block in <top (required)>'
jekyll_1 | 43: from /usr/gem/gems/bundler-2.3.9/lib/bundler/cli.rb:25:in `start'
jekyll_1 | 42: from /usr/gem/gems/bundler-2.3.9/lib/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
jekyll_1 | 41: from /usr/gem/gems/bundler-2.3.9/lib/bundler/cli.rb:31:in `dispatch'
jekyll_1 | 40: from /usr/gem/gems/bundler-2.3.9/lib/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
jekyll_1 | 39: from /usr/gem/gems/bundler-2.3.9/lib/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
jekyll_1 | 38: from /usr/gem/gems/bundler-2.3.9/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
jekyll_1 | 37: from /usr/gem/gems/bundler-2.3.9/lib/bundler/cli.rb:483:in `exec'
jekyll_1 | 36: from /usr/gem/gems/bundler-2.3.9/lib/bundler/cli/exec.rb:23:in `run'
jekyll_1 | 35: from /usr/gem/gems/bundler-2.3.9/lib/bundler/cli/exec.rb:58:in `kernel_load'
jekyll_1 | 34: from /usr/gem/gems/bundler-2.3.9/lib/bundler/cli/exec.rb:58:in `load'
jekyll_1 | 33: from /usr/gem/bin/jekyll:23:in `<top (required)>'
jekyll_1 | 32: from /usr/gem/bin/jekyll:23:in `load'
jekyll_1 | 31: from /usr/gem/gems/jekyll-3.9.0/exe/jekyll:15:in `<top (required)>'
jekyll_1 | 30: from /usr/gem/gems/mercenary-0.3.6/lib/mercenary.rb:19:in `program'
jekyll_1 | 29: from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/program.rb:42:in `go'
jekyll_1 | 28: from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `execute'
jekyll_1 | 27: from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `each'
jekyll_1 | 26: from /usr/gem/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `block in execute'
jekyll_1 | 25: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/commands/serve.rb:75:in `block (2 levels) in init_with_program'
jekyll_1 | 24: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/commands/serve.rb:93:in `start'
jekyll_1 | 23: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/commands/serve.rb:93:in `each'
jekyll_1 | 22: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/commands/serve.rb:93:in `block in start'
jekyll_1 | 21: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/commands/build.rb:36:in `process'
jekyll_1 | 20: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/commands/build.rb:65:in `build'
jekyll_1 | 19: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/command.rb:28:in `process_site'
jekyll_1 | 18: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/site.rb:71:in `process'
jekyll_1 | 17: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/site.rb:192:in `render'
jekyll_1 | 16: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/site.rb:471:in `render_pages'
jekyll_1 | 15: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/site.rb:471:in `each'
jekyll_1 | 14: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/site.rb:472:in `block in render_pages'
jekyll_1 | 13: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/site.rb:479:in `render_regenerated'
jekyll_1 | 12: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/renderer.rb:62:in `run'
jekyll_1 | 11: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/renderer.rb:79:in `render_document'
jekyll_1 | 10: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/renderer.rb:121:in `render_liquid'
jekyll_1 | 9: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/liquid_renderer/file.rb:12:in `parse'
jekyll_1 | 8: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/liquid_renderer/file.rb:49:in `measure_time'
jekyll_1 | 7: from /usr/gem/gems/jekyll-3.9.0/lib/jekyll/liquid_renderer/file.rb:13:in `block in parse'
jekyll_1 | 6: from /usr/gem/gems/liquid-4.0.3/lib/liquid/template.rb:116:in `parse'
jekyll_1 | 5: from /usr/gem/gems/liquid-4.0.3/lib/liquid/template.rb:132:in `parse'
jekyll_1 | 4: from /usr/gem/gems/liquid-4.0.3/lib/liquid/document.rb:5:in `parse'
jekyll_1 | 3: from /usr/gem/gems/liquid-4.0.3/lib/liquid/document.rb:10:in `parse'
jekyll_1 | 2: from /usr/gem/gems/liquid-4.0.3/lib/liquid/block_body.rb:39:in `parse'
jekyll_1 | 1: from /usr/gem/gems/liquid-4.0.3/lib/liquid/block_body.rb:128:in `create_variable'
jekyll_1 | /usr/gem/gems/liquid-4.0.3/lib/liquid/block_body.rb:136:in `raise_missing_variable_terminator': Liquid syntax error (line 851): Variable '{{length}' was not properly terminated with regexp: /\\}\\}/ (Liquid::SyntaxError)