unable to sign in to Garmin connect mobile app

Did you try removing and reinstalling the application?

Try with another phone (i.e. of a friend, an old phone of yours, or even a phone in a store). If it works on another phone, the problem is with your phone. For example you can try clearing the accounts from Android's Account Manager - looks like there may be some broken one, or perhpas some application conflcting with the standard functionality. 

If you encouter the same problems on the other phone, there may be a problem with the account. In that case create a new one.

The problem started after the application was updated on February 4th

Unknown said:

The problem started after the application was updated on February 4th

Possible, but since nobody else complains about it, it is not something general, but quite specifical to your situation, hence if you want to fix it, you need to find the culprit.. You simply need to localize where the conflict comes from.  The steps I suggested, can help you with it.

From the error message it is clearly an issue with the Account manager, however there is no Garmin app showing in the account manager that can be deleted. Also if you try to add an account, Garmin does not appear amongst the app options to choose from, hence the 'addAccountExplicitly returned False'? Uninstalling and re-installing does not work. Yes I can sign into the account on another device(pc) but I need it to work on my phone. Whatever changed was as a result of the update 4th Feb 2020.

Unknown said:

Whatever changed was as a result of the update 4th Feb 2020.

As I wrote, it is possible, and it also may be just a concidence that it got blocked during the upgrade. However, your case is apparently exceptional, hence it is clear it somehow relates to your phone, its settings, OS version, or installed applications. Unless you start localizing it, and excluding one possibility after another, you won't know and wont't be able to fix it. And we, other users, here on the forum, cannot help you, since we do not have such problems and cannot replicate your issue.

So unless you want to simpy wait till it fixes alone (which may never happen), you'll need to be pro-active and try to find the conflict. Starting with another phone of the same brand and OS version, would be the best. If you could perform (without too much hassle) a factory reset of your phone, it would be another possibility. But if you are not willing to search a solution, I am afraid nobody else will do it for you. Of course, you can also try calling Garmin's support - perhaps they already have encountered such cases, and can help, but I would not keep the hopes too high.