That issue has nothing to do with the keyboard, it's a glitch within the app itself. Contact the developer of the app

 

Go to settings, apps, find your keyboard app, choose open...

 

Some will be listed and some will not, depending on the manufacturer...
You may be able to try settings, language and input, and find the keyboard from there...

 

If you have a Pixel 4a then your built-in keyboard app is called "GBoard" - Google's keyboard app.

Everything in the OS that you interact with is an app: your home screens and app drawer are provided by an app called a "launcher" (in your case "Pixel Launcher"), your system settings are viewed and changed by an app called "Settings", etc. Many of these are the manufacturer's own apps rather than a core part of the OS (including the launcher, keyboard, message app, etc), and many of these can be replaced by an alternative of your choice.

Sometimes a particular app doesn't play well with a particular keyboard, though given that GBoard is one of the most widely-used keyboard apps it would be spectacularly sloppy of the developer not to have tested that. So you could try installing a different app and see whether it makes a difference. But I just tried installing the app on my phone and tried with 3 different keyboard apps and it made no difference, so I believe that the problem is with the app rather than any particular keyboard.

I don't know of a way of just bringing the keyboard up regardless, but it might not help even if you could. If the problem is that the app is failing to request input, and hence the keyboard is not appearing because no request for one has been made, then just opening the keyboard won't necessarily help.

 

I am wondering if a bluetooth keyboard could be a workaround, mostly because I just got one.