PPI to DPI?!?

There is a "size trap" when exporting ID files to raster formats such as png. I've written about it elsewhere and may help: https://indesignsecrets.com/size-trap-exporting-jpgs-on-screen.php

Sorry, I missed that you are exporting out of InDesign. Is the PNG for print output?

One thing to note when you save in a web format like PNG or JPG out of Photoshop—if you use Export, or the legacy Save for Web, the resolution metadata gets stripped and the resolution defaults to 72ppi. The pixel dimensions don't change, but the output dimensions change accordingly. So don’t use Export in Photoshop, use Save As instead.

Hi Brandon: Sounds reasonable. If we can be of further assistance, just let us know.

~Barb

Can you be specific as to where you are exporting your PNG to? Are you exporting to a pdf or as another file format such as a jpeg? Just who or what is rejecting your export? As the others here have said the dpi/ppi thing just doesn't fly.

Hi Brandon: Everyone is correct with their definitions (Tl;dr: ppi defines image resolution, dpi defines print resolution and the two are mixed up all the time) but I think John is on point with his suspicion that someone did a bad job writing the error message. 

What site are you trying to upload to, and will you show a screen shot of the error message?

~Barb

As you've discovered, they're two different measurements, but sadly, reference to ppi and dpi are commonly used interchangeably to refer to image resolution. Your image isn't being rejected because of its "dpi". That's just someone having mis-wrote the rejection message, misusing the term.

You may open the PNG image in Photoshop too, in Image size, you can check the Dimensions, that are expressed in pixels.

Depending on the resolution used (ppi) the Width and Height change, but it is irrelevant here. Likely the image is rejected because the Size or the dimensions, total number of pixels is higher than allowed.

In Photoshop and also in Indesign you can check that value in Image>Image size in PS. In ID open Links Panel (Cmd/ctrl+Shift+D) and read the Dimensions values, expressed in pixels.

PPI is the correct term to use for image resolution -- what rejection message do you receive when "uploading"?
If your image is being used on say a website the resolution is irrelevant, as the image will match the resolution of the monitor. 

As a matter of interest, why do you think you need DPI – it stands for Dots per Inch and refers to the dots printers' print (not half-tones screens, which are measured in LPI, Lines Per Inch).

Resolution is only important when you're producing content for printing. Rule of thumb for images is 300PPI which allows for two pixels per halftone dot. 150LPI is a good halftone screen size for good quality litho printing on coated paper.

Are you sure it's being rejected because it's ppi, and not for a different reason?

DPI is dots-per-inch, which is a measurment for the density of halftone dots of ink on paper. PPI is pixels-per-inch, which is what you see in Photoshop, and what you see in InDesign when you look at the effective resolution (because as you enlarge the size of a pixel-based image, the number of pixels-per-inch goes down, and it goes up if you shrink an image). Most people who say dpi when talking about what goes on in the computer really mean ppi.