Please note that this is a Community with mainly volunteers. They(/we) can come and go as they(/we) please and are not obliged to help 24/7. So it might be you have to wait a little while for a response. Please try to be patient.
Also, I think it's very hard to help you with this issue, as we don't exactly know what "PowerApps" actually requires as a PFX. Let's Encrypt simply delivers plain PEM files. I have no clue what openssl pkcs12 is correct for PowerApps.
Also note that openssl pkcs12 can do more than just put a PEM file which contains everything into a PFX file. It probably wants to have the end leaf certificate, chain and private key separately as options. See /docs/man1.1.1/man1/openssl-pkcs12.html for more info.