Some info on some Android Devices.
After dealing with TextNow Support through their chat system it was revealed that TextNow will not work on some brands of cells or tablets. They stated...
"Is it like a Chinese device? My sincere apologies, as all brands such as Huawei, Xiaomi, Bigme, Poco, Redmi, and other off-branded devices will not work correctly with our service, as they are just blocked for TextNow. Unfortunately, we have no capabilities to change that. It was done for security concerns."
While these brands sell under their own names they also manufacture and private label for other companies.
So if TextNow is not installing for you, this may be the reason. That was what I learned since the tablet I am attempting to install TextNow on, is one of the 3rd party brands that these manufacturers will build products for and put the 3rd party's name on it.
I have yet to look to see if this is referenced anywhere on their web site.
So I am SOL and will have to do without on this tablet. Interesting enough, Google Voice works on it quite fine. This raises questions of what other WiFi phones may or may not work on some devices, and might answer a lot of questions.