The car is running late, it's nothing that hasn't happened before to pretty much every team. Ferrari on occasion turned up with the new car several races into the new season.
The aero and tyre changes means that it's a little more important to get a handle on things quickly this year.
Bringing their best/new car into the season is entirely different to bringing no car to testing.
Ultimately you can bolt a front/rear wing onto last year's car and go around if you have all the rest of the stuff you need.
On the other hand you often have situations where teams say yup, we're totally going to be there, just a day late, then it's another day, then the third they say there is an issue, then they suddenly turn up and it's announced they have a new title sponsor and were actually hours away from being unable to pay bills and would have cars repossessed. Other times teams fold while they are putting a brave face on it trying to make themselves seem better to sponsors.
Other times it's purely just a delay in parts. The real issue though again is why aren't they just running the test parts they had last year. A bunch of cars including Williams iirc, ran prototype wings in testing last year so why aren't they just running that car.
I'm not saying Williams are about to fold but a lot of times teams are in financial trouble and about to fold this is often how these things play out.
two things to think about, if it's not financial they just put any car on track and give their drivers experience, why wouldn't they?
The other is, WCC, Stroll, Martini, they've lost a lot of their income compared to last year. They've brought on Rokit but I haven't even seen a rough guess at how much they got it for. It's quite possible Williams are literally 10s of millions down compared to last season and I really wouldn't be surprised if they were having financial issues.
There is also a rumour supposedly from an insider that says their internal simulations put this year's car 2 seconds slower than last year's car. Their aero department is a complete disaster so if they had to break what speed they had with the new wings and couldn't find improvements, it's worrying.