Normally if that happens, just save what you're going to post, and set as the background on a new one then If that doesn't work, you've got something "inappropriate" in your description or text on the collage (Like Pic Collage text, if you used something like Phonto for text then that's fine) it won't let you post, so try holding down a letter and change the letter to something special like "ë" for example, Pic collage also thinks "watchéd" is "inappropriate," ((or that might just be for me))so that might be what's up, otherwise, I'm not really sure what is happening for you

It does that when you say words

Watched is not a bad word with me

It's just Pastel

^Rip me lol