Can't resize APFS Container: Not enough free space in Container due to APFS limits or tidemarks

Solution 1:

This is the key part of the error message:

(perhaps caused by APFS Snapshot usage by Time Machine)

You must remove the local snapshots. First, list them:

$ tmutil listlocalsnapshots /

You may wish / need to delete local snapshots one by one from taking the date from the listing above:

$ tmutil deletelocalsnapshots 2018-07-14-211120

You can also thin them based on the amount of space you wish to reclaim by asking for a lot of space back:

$ tmutil thinlocalsnapshots / 9999999999999

Then, you can resize your APFS partition:

$ sudo diskutil apfs resizeContainer disk0s2 750g

Solution 2:

This should unlock the ability to resize your disk:

tmutil listlocalsnapshots / \
  | cut -d. -f4 \
  | xargs -n1 tmutil deletelocalsnapshots

It does the same thing as Neil's answer, except that it copy-pastes the timestamps for you.