That is because you didn't follow the steps:
https://proofingtoolgui.org/en_installing.html

And the #4:
https://proofingtoolgui.org/faq.html#4

The information you provide is not enough to analyse your problem.

EDIT (2020-06-13):
Send me an e-mail so that we can see how to solve it:
marcoagpinto @ sapo.pt

Hello!

I have been trying to find out the answer.

I asked in the add-ons riot channel about it but no one replied.

I found this URL:
https://support.mozilla.org/en-US/questions/1027778

It says there that Android doesn't accept dictionaries, which is strange.

I tried to install LanguageTool which uses my dictionary, and it doesn't work on Android.

It is terrible :-(

:-)

There is still a long way to go... but we will get there.

Hello dear brother,

Don't worry, follow the install instructions here:
https://proofingtoolgui.org/en_installing.html

If you are using Firefox 59 or older, download the legacy add-on:
https://github.com/marcoagpinto/aoo-mozilla-en-dict

Tell me if it worked.

Kind regards,

Thank you my dear brother,

I have been dedicating the latest times to add possessives and plurals to words.

I have been analysing word by word to see which ones are nouns and thus add plural+possessive.

I still have issues with the "mass nouns"/"uncountable nouns" since only some accept plurals and I am not sure if they all accept possessives.

I committed the current version to Gerrit and it will come with LibreOffice 6.4 in January, but then I found out that some more common plurals were missing, such as "punishers", which I will come in December's version but it will only be integrated into LibreOffice in May (they only integrate the spellers twice a year: May and November).

There is still a long way to go and I know that I can make the difference. Like Spock once said: "because the needs of the many outweigh the needs of the one or the few". Individual effort will benefit millions.

Kind regards from,
> Marco A.G.Pinto
------------------