It is a known issue in 12.1.1 version of Premiere Pro and 10.11.X version of the MAC OS where MP4 or MOV files containing H.264 codec import as Audio only.
To resolve this issue, update MAC OS to 10.12 or above, clear the Media Cache and restart the machine.
Adobe has depreciated some QuickTime 32-bit codecs. Check if the codec you are using is listed in this document: Dropped support for QuickTime 7 era formats and codecs. If it is listed, then transcode the files to some newer codec or format and then try importing the files.
If the codecs are not listed in the above document then try the following instructions:
Check the source of the file (for example if it is downloaded from the internet, sent by someone, or recorded using a mobile). Check your file codec and format information using Media Info. Choose File > Import. Once the file is imported, go to View menu and select Tree view.
For more information on supported file formats, see Supported file formats.
If the codec or format is not listed in the supported file formats list or the MTS file does not have a proper folder structure, then try to transcode the files to one of the supported formats using any 3rd-party tool that converts videos from one format to another.
If the files are supported and are not listed in the Dropped support for QuickTime 7 era formats and codecs document, then try the steps below:
Rename the file(s) from MOV to MP4 or MTS to M2TS and then try to import the file. If that does not work, try to Activate Root user on MAC or Hidden Admin on Windows and try to import the file. If this also does not work, log into Safe mode on Windows or Safe mode in MAC and try to import the files. This procedure also ensures that any third-party software is not blocking the file import.